Tests python3.11 compat

Co-authored-by: Richard Schreiber <schreiber@rami.io>

doc/api/resources/categories.rst

@@ -23,6 +23,22 @@ position                              integer                    An integer, use
 is_addon                              boolean                    If ``true``, items within this category are not on sale
                                                                  on their own but the category provides a source for
                                                                  defining add-ons for other products.
+cross_selling_mode                    string                     If ``null``, cross-selling is disabled for this category.
+                                                                 If ``"only"``, it is only visible in the cross-selling
+                                                                 step.
+                                                                 If ``"both"``, it is visible on the normal index page
+                                                                 as well.
+                                                                 Only available if ``is_addon`` is ``false``.
+cross_selling_condition               string                     Only relevant if ``cross_selling_mode`` is not ``null``.
+                                                                 If ``"always"``, always show in cross-selling step.
+                                                                 If ``"products"``, only show if the cart contains one of
+                                                                 the products listed in ``cross_selling_match_products``.
+                                                                 If ``"discounts"``, only show products that qualify for
+                                                                 a discount according to discount rules.
+cross_selling_match_products          list of integer            Only relevant if ``cross_selling_condition`` is
+                                                                 ``"products"``. Internal ID of the items of which at
+                                                                 least one needs to be in the cart for this category to
+                                                                 be shown.
 ===================================== ========================== =======================================================
 
 
@@ -60,7 +76,10 @@ Endpoints
             "internal_name": "",
             "description": {"en": "Tickets are what you need to get in."},
             "position": 1,
-            "is_addon": false
+            "is_addon": false,
+            "cross_selling_mode": null,
+            "cross_selling_condition": null,
+            "cross_selling_match_products": []
           }
         ]
       }
@@ -102,7 +121,10 @@ Endpoints
         "internal_name": "",
         "description": {"en": "Tickets are what you need to get in."},
         "position": 1,
-        "is_addon": false
+        "is_addon": false,
+        "cross_selling_mode": null,
+        "cross_selling_condition": null,
+        "cross_selling_match_products": []
       }
 
    :param organizer: The ``slug`` field of the organizer to fetch
@@ -130,7 +152,10 @@ Endpoints
         "internal_name": "",
         "description": {"en": "Tickets are what you need to get in."},
         "position": 1,
-        "is_addon": false
+        "is_addon": false,
+        "cross_selling_mode": null,
+        "cross_selling_condition": null,
+        "cross_selling_match_products": []
       }
 
    **Example response**:
@@ -147,7 +172,10 @@ Endpoints
         "internal_name": "",
         "description": {"en": "Tickets are what you need to get in."},
         "position": 1,
-        "is_addon": false
+        "is_addon": false,
+        "cross_selling_mode": null,
+        "cross_selling_condition": null,
+        "cross_selling_match_products": []
       }
 
    :param organizer: The ``slug`` field of the organizer of the event to create a category for
@@ -193,7 +221,10 @@ Endpoints
         "internal_name": "",
         "description": {"en": "Tickets are what you need to get in."},
         "position": 1,
-        "is_addon": true
+        "is_addon": true,
+        "cross_selling_mode": null,
+        "cross_selling_condition": null,
+        "cross_selling_match_products": []
       }
 
    :param organizer: The ``slug`` field of the organizer to modify

src/pretix/api/serializers/item.py

@@ -441,7 +441,22 @@ class ItemCategorySerializer(I18nAwareModelSerializer):
 
     class Meta:
         model = ItemCategory
-        fields = ('id', 'name', 'internal_name', 'description', 'position', 'is_addon')
+        fields = (
+            'id', 'name', 'internal_name', 'description', 'position',
+            'is_addon', 'cross_selling_mode',
+            'cross_selling_condition', 'cross_selling_match_products'
+        )
+
+    def validate(self, data):
+        data = super().validate(data)
+
+        full_data = self.to_internal_value(self.to_representation(self.instance)) if self.instance else {}
+        full_data.update(data)
+
+        if full_data.get('is_addon') and full_data.get('cross_selling_mode'):
+            raise ValidationError('is_addon and cross_selling_mode are mutually exclusive')
+
+        return data
 
 
 class QuestionOptionSerializer(I18nAwareModelSerializer):

src/pretix/base/migrations/0271_itemcategory_cross_selling.py

@@ -0,0 +1,32 @@
+# Generated by Django 4.2.11 on 2024-05-27 13:19
+
+from django.db import migrations, models
+
+import pretix.base.models.orders
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("pretixbase", "0270_historicpassword"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="itemcategory",
+            name="cross_selling_condition",
+            field=models.CharField(null=True, max_length=10),
+        ),
+        migrations.AddField(
+            model_name="itemcategory",
+            name="cross_selling_mode",
+            field=models.CharField(null=True, max_length=5),
+        ),
+        migrations.AddField(
+            model_name="itemcategory",
+            name="cross_selling_match_products",
+            field=models.ManyToManyField(
+                related_name="matched_by_cross_selling_categories", to="pretixbase.item"
+            ),
+        ),
+    ]

src/pretix/base/models/discount.py

@@ -20,11 +20,11 @@
 # <https://www.gnu.org/licenses/>.
 #
 
-from collections import defaultdict
+from collections import defaultdict, namedtuple
 from decimal import Decimal
 from itertools import groupby
-from math import ceil
-from typing import Dict, Optional, Tuple
+from math import ceil, inf
+from typing import Dict
 
 from django.core.exceptions import ValidationError
 from django.core.validators import MinValueValidator
@@ -36,6 +36,8 @@ from django_scopes import ScopedManager
 from pretix.base.decimal import round_decimal
 from pretix.base.models.base import LoggedModel
 
+PositionInfo = namedtuple('PositionInfo', ['item_id', 'subevent_id', 'line_price_gross', 'is_addon_to', 'voucher_discount'])
+
 
 class Discount(LoggedModel):
     SUBEVENT_MODE_MIXED = 'mixed'
@@ -245,22 +247,26 @@ class Discount(LoggedModel):
             return False
         return True
 
-    def _apply_min_value(self, positions, condition_idx_group, benefit_idx_group, result):
-        if self.condition_min_value and sum(positions[idx][2] for idx in condition_idx_group) < self.condition_min_value:
+    def _apply_min_value(self, positions, condition_idx_group, benefit_idx_group, result, collect_potential_discounts, subevent_id):
+        if self.condition_min_value and sum(positions[idx].line_price_gross for idx in condition_idx_group) < self.condition_min_value:
             return
 
         if self.condition_min_count or self.benefit_only_apply_to_cheapest_n_matches:
             raise ValueError('Validation invariant violated.')
 
         for idx in benefit_idx_group:
-            previous_price = positions[idx][2]
+            previous_price = positions[idx].line_price_gross
             new_price = round_decimal(
                 previous_price * (Decimal('100.00') - self.benefit_discount_matching_percent) / Decimal('100.00'),
                 self.event.currency,
             )
             result[idx] = new_price
 
-    def _apply_min_count(self, positions, condition_idx_group, benefit_idx_group, result):
+        if collect_potential_discounts is not None:
+            for idx in condition_idx_group:
+                collect_potential_discounts[idx] = [(self, inf, -1, subevent_id)]
+
+    def _apply_min_count(self, positions, condition_idx_group, benefit_idx_group, result, collect_potential_discounts, subevent_id):
         if len(condition_idx_group) < self.condition_min_count:
             return
 
@@ -268,23 +274,53 @@ class Discount(LoggedModel):
             raise ValueError('Validation invariant violated.')
 
         if self.benefit_only_apply_to_cheapest_n_matches:
-            if not self.condition_min_count:
-                raise ValueError('Validation invariant violated.')
-
-            condition_idx_group = sorted(condition_idx_group, key=lambda idx: (positions[idx][2], -idx))  # sort by line_price
-            benefit_idx_group = sorted(benefit_idx_group, key=lambda idx: (positions[idx][2], -idx))  # sort by line_price
+            # sort by line_price
+            condition_idx_group = sorted(condition_idx_group, key=lambda idx: (positions[idx].line_price_gross, -idx))
+            benefit_idx_group = sorted(benefit_idx_group, key=lambda idx: (positions[idx].line_price_gross, -idx))
 
             # Prevent over-consuming of items, i.e. if our discount is "buy 2, get 1 free", we only
             # want to match multiples of 3
-            n_groups = min(len(condition_idx_group) // self.condition_min_count, ceil(len(benefit_idx_group) / self.benefit_only_apply_to_cheapest_n_matches))
+
+            # how many discount applications are allowed according to condition products in cart
+            possible_applications_cond = len(condition_idx_group) // self.condition_min_count
+
+            # how many discount applications are possible according to benefitting products in cart
+            possible_applications_benefit = ceil(len(benefit_idx_group) / self.benefit_only_apply_to_cheapest_n_matches)
+
+            n_groups = min(possible_applications_cond, possible_applications_benefit)
             consume_idx = condition_idx_group[:n_groups * self.condition_min_count]
             benefit_idx = benefit_idx_group[:n_groups * self.benefit_only_apply_to_cheapest_n_matches]
+
+            if collect_potential_discounts is not None:
+                if n_groups * self.benefit_only_apply_to_cheapest_n_matches > len(benefit_idx_group):
+                    # partially used discount ("for each 1 ticket you buy, get 50% on 2 t-shirts", cart content: 1 ticket
+                    # but only 1 t-shirt) -> 1 shirt definitiv potential discount
+                    for idx in consume_idx:
+                        collect_potential_discounts[idx] = [
+                            (self, n_groups * self.benefit_only_apply_to_cheapest_n_matches - len(benefit_idx_group), -1, subevent_id)
+                        ]
+
+                if possible_applications_cond * self.benefit_only_apply_to_cheapest_n_matches > len(benefit_idx_group):
+                    # unused discount ("for each 1 ticket you buy, get 50% on 2 t-shirts", cart content: 1 ticket
+                    # but 0 t-shirts) -> 2 shirt maybe potential discount (if the 1 ticket is not consumed by a later discount)
+                    for i, idx in enumerate(condition_idx_group[
+                                            n_groups * self.condition_min_count:
+                                            possible_applications_cond * self.condition_min_count
+                                            ]):
+                        collect_potential_discounts[idx] += [
+                            (self, self.benefit_only_apply_to_cheapest_n_matches, i // self.condition_min_count, subevent_id)
+                        ]
+
         else:
             consume_idx = condition_idx_group
             benefit_idx = benefit_idx_group
 
+            if collect_potential_discounts is not None:
+                for idx in consume_idx:
+                    collect_potential_discounts[idx] = [(self, inf, -1, subevent_id)]
+
         for idx in benefit_idx:
-            previous_price = positions[idx][2]
+            previous_price = positions[idx].line_price_gross
             new_price = round_decimal(
                 previous_price * (Decimal('100.00') - self.benefit_discount_matching_percent) / Decimal('100.00'),
                 self.event.currency,
@@ -292,15 +328,16 @@ class Discount(LoggedModel):
             result[idx] = new_price
 
         for idx in consume_idx:
-            result.setdefault(idx, positions[idx][2])
+            result.setdefault(idx, positions[idx].line_price_gross)
 
-    def apply(self, positions: Dict[int, Tuple[int, Optional[int], Decimal, bool, Decimal]]) -> Dict[int, Decimal]:
+    def apply(self, positions: Dict[int, PositionInfo],
+              collect_potential_discounts=None) -> Dict[int, Decimal]:
         """
         Tries to apply this discount to a cart
 
-        :param positions: Dictionary mapping IDs to tuples of the form
-                          ``(item_id, subevent_id, line_price_gross, is_addon_to, voucher_discount)``.
+        :param positions: Dictionary mapping IDs to PositionInfo tuples.
                           Bundled positions may not be included.
+        :param collect_potential_discounts: For detailed description, see pretix.base.services.pricing.apply_discounts
 
         :return: A dictionary mapping keys from the input dictionary to new prices. All positions
                  contained in this dictionary are considered "consumed" and should not be considered
@@ -342,13 +379,13 @@ class Discount(LoggedModel):
 
         if self.subevent_mode == self.SUBEVENT_MODE_MIXED:  # also applies to non-series events
             if self.condition_min_count:
-                self._apply_min_count(positions, condition_candidates, benefit_candidates, result)
+                self._apply_min_count(positions, condition_candidates, benefit_candidates, result, collect_potential_discounts, None)
             else:
-                self._apply_min_value(positions, condition_candidates, benefit_candidates, result)
+                self._apply_min_value(positions, condition_candidates, benefit_candidates, result, collect_potential_discounts, None)
 
         elif self.subevent_mode == self.SUBEVENT_MODE_SAME:
             def key(idx):
-                return positions[idx][1] or 0  # subevent_id
+                return positions[idx].subevent_id or 0
 
             # Build groups of candidates with the same subevent, then apply our regular algorithm
             # to each group
@@ -357,11 +394,11 @@ class Discount(LoggedModel):
             candidate_groups = [(k, list(g)) for k, g in _groups]
 
             for subevent_id, g in candidate_groups:
-                benefit_g = [idx for idx in benefit_candidates if positions[idx][1] == subevent_id]
+                benefit_g = [idx for idx in benefit_candidates if positions[idx].subevent_id == subevent_id]
                 if self.condition_min_count:
-                    self._apply_min_count(positions, g, benefit_g, result)
+                    self._apply_min_count(positions, g, benefit_g, result, collect_potential_discounts, subevent_id)
                 else:
-                    self._apply_min_value(positions, g, benefit_g, result)
+                    self._apply_min_value(positions, g, benefit_g, result, collect_potential_discounts, subevent_id)
 
         elif self.subevent_mode == self.SUBEVENT_MODE_DISTINCT:
             if self.condition_min_value or not self.benefit_same_products:
@@ -377,9 +414,9 @@ class Discount(LoggedModel):
             # Build a list of subevent IDs in descending order of frequency
             subevent_to_idx = defaultdict(list)
             for idx, p in positions.items():
-                subevent_to_idx[p[1]].append(idx)
+                subevent_to_idx[p.subevent_id].append(idx)
             for v in subevent_to_idx.values():
-                v.sort(key=lambda idx: positions[idx][2])
+                v.sort(key=lambda idx: positions[idx].line_price_gross)
             subevent_order = sorted(list(subevent_to_idx.keys()), key=lambda s: len(subevent_to_idx[s]), reverse=True)
 
             # Build groups of exactly condition_min_count distinct subevents
@@ -394,7 +431,7 @@ class Discount(LoggedModel):
                     l = [ll for ll in l if ll in condition_candidates and ll not in current_group]
                     if cardinality and len(l) != cardinality:
                         continue
-                    if se not in {positions[idx][1] for idx in current_group}:
+                    if se not in {positions[idx].subevent_id for idx in current_group}:
                         candidates += l
                         cardinality = len(l)
 
@@ -403,7 +440,7 @@ class Discount(LoggedModel):
 
                 # Sort the list by prices, then pick one. For "buy 2 get 1 free" we apply a "pick 1 from the start
                 # and 2 from the end" scheme to optimize price distribution among groups
-                candidates = sorted(candidates, key=lambda idx: positions[idx][2])
+                candidates = sorted(candidates, key=lambda idx: positions[idx].line_price_gross)
                 if len(current_group) < (self.benefit_only_apply_to_cheapest_n_matches or 0):
                     candidate = candidates[0]
                 else:
@@ -415,14 +452,14 @@ class Discount(LoggedModel):
                 if len(current_group) >= max(self.condition_min_count, 1):
                     candidate_groups.append(current_group)
                     for c in current_group:
-                        subevent_to_idx[positions[c][1]].remove(c)
+                        subevent_to_idx[positions[c].subevent_id].remove(c)
                     current_group = []
 
             # Distribute "leftovers"
             for se in subevent_order:
                 if subevent_to_idx[se]:
                     for group in candidate_groups:
-                        if se not in {positions[idx][1] for idx in group}:
+                        if se not in {positions[idx].subevent_id for idx in group}:
                             group.append(subevent_to_idx[se].pop())
                             if not subevent_to_idx[se]:
                                 break
@@ -432,6 +469,8 @@ class Discount(LoggedModel):
                     positions,
                     [idx for idx in g if idx in condition_candidates],
                     [idx for idx in g if idx in benefit_candidates],
-                    result
+                    result,
+                    None,
+                    None
                 )
         return result

src/pretix/base/models/event.py

@@ -870,10 +870,12 @@ class Event(EventMixin, LoggedModel):
         for i in Item.objects.filter(event=other).prefetch_related(
             'variations', 'limit_sales_channels', 'require_membership_types',
             'variations__limit_sales_channels', 'variations__require_membership_types',
+            'matched_by_cross_selling_categories',
         ):
             vars = list(i.variations.all())
             require_membership_types = list(i.require_membership_types.all())
             limit_sales_channels = list(i.limit_sales_channels.all())
+            matched_by_cross_selling_categories = list(i.matched_by_cross_selling_categories.all())
             item_map[i.pk] = i
             i.pk = None
             i.event = self
@@ -911,6 +913,9 @@ class Event(EventMixin, LoggedModel):
                 if not v.all_sales_channels:
                     v.limit_sales_channels.set(self.organizer.sales_channels.filter(identifier__in=[s.identifier for s in limit_sales_channels]))
 
+            if matched_by_cross_selling_categories:
+                i.matched_by_cross_selling_categories.set([category_map[c.pk] for c in matched_by_cross_selling_categories])
+
         for i in self.items.filter(hidden_if_item_available__isnull=False):
             i.hidden_if_item_available = item_map[i.hidden_if_item_available_id]
             i.save()

src/pretix/base/models/items.py

@@ -63,14 +63,13 @@ from django_countries.fields import Country
 from django_scopes import ScopedManager
 from i18nfield.fields import I18nCharField, I18nTextField
 
+from pretix.base.media import MEDIA_TYPES
+from pretix.base.models import Event, SubEvent
 from pretix.base.models.base import LoggedModel
 from pretix.base.models.fields import MultiStringField
 from pretix.base.models.tax import TaxedPrice
 from pretix.base.timemachine import time_machine_now
-
-from ...helpers.images import ImageSizeValidator
-from ..media import MEDIA_TYPES
-from .event import Event, SubEvent
+from pretix.helpers.images import ImageSizeValidator
 
 
 class ItemCategory(LoggedModel):
@@ -111,6 +110,33 @@ class ItemCategory(LoggedModel):
                     'only be bought in combination with a product that has this category configured as a possible '
                     'source for add-ons.')
     )
+    CROSS_SELLING_MODES = (
+        (None, _('Normal category')),
+        ('both', _('Normal + cross-selling category')),
+        ('only', _('Cross-selling category')),
+    )
+    cross_selling_mode = models.CharField(
+        choices=CROSS_SELLING_MODES,
+        null=True,
+        max_length=5
+    )
+    CROSS_SELLING_CONDITION = (
+        ('always', _('Always show in cross-selling step')),
+        ('discounts', _('Only show products that qualify for a discount according to discount rules')),
+        ('products', _('Only show if the cart contains one of the following products')),
+    )
+    cross_selling_condition = models.CharField(
+        verbose_name=_("Cross-selling condition"),
+        choices=CROSS_SELLING_CONDITION,
+        null=True,
+        max_length=10,
+    )
+    cross_selling_match_products = models.ManyToManyField(
+        'pretixbase.Item',
+        blank=True,
+        verbose_name=_("Cross-selling condition products"),
+        related_name="matched_by_cross_selling_categories",
+    )
 
     class Meta:
         verbose_name = _("Product category")
@@ -119,19 +145,31 @@ class ItemCategory(LoggedModel):
 
     def __str__(self):
         name = self.internal_name or self.name
-        if self.is_addon:
-            return _('{category} (Add-On products)').format(category=str(name))
+        category_type = self.get_category_type_display()
+        if category_type:
+            return _('{category} ({category_type})').format(category=str(name), category_type=category_type)
         return str(name)
 
     def get_category_type_display(self):
         if self.is_addon:
-            return _('Add-On products')
+            return _('Add-on category')
+        elif self.cross_selling_mode:
+            return self.get_cross_selling_mode_display()
         else:
             return None
 
     @property
     def category_type(self):
-        return 'addon' if self.is_addon else 'normal'
+        return 'addon' if self.is_addon else self.cross_selling_mode or 'normal'
+
+    @category_type.setter
+    def category_type(self, new_value):
+        if new_value == 'addon':
+            self.is_addon = True
+            self.cross_selling_mode = None
+        else:
+            self.is_addon = False
+            self.cross_selling_mode = None if new_value == 'normal' else new_value
 
     def delete(self, *args, **kwargs):
         super().delete(*args, **kwargs)
@@ -270,7 +308,7 @@ class SubEventItemVariation(models.Model):
         return True
 
 
-def filter_available(qs, channel='web', voucher=None, allow_addons=False):
+def filter_available(qs, channel='web', voucher=None, allow_addons=False, allow_cross_sell=False):
     # Channel can currently be a SalesChannel or a str, since we need that compatibility, but a SalesChannel
     # makes the query SIGNIFICANTLY faster
     from .organizer import SalesChannel
@@ -291,6 +329,8 @@ def filter_available(qs, channel='web', voucher=None, allow_addons=False):
 
     if not allow_addons:
         q &= Q(Q(category__isnull=True) | Q(category__is_addon=False))
+    if not allow_cross_sell:
+        q &= Q(Q(category__isnull=True) | ~Q(category__cross_selling_mode='only'))
 
     if voucher:
         if voucher.item_id:
@@ -304,8 +344,8 @@ def filter_available(qs, channel='web', voucher=None, allow_addons=False):
 
 
 class ItemQuerySet(models.QuerySet):
-    def filter_available(self, channel='web', voucher=None, allow_addons=False):
-        return filter_available(self, channel, voucher, allow_addons)
+    def filter_available(self, channel='web', voucher=None, allow_addons=False, allow_cross_sell=False):
+        return filter_available(self, channel, voucher, allow_addons, allow_cross_sell)
 
 
 class ItemQuerySetManager(ScopedManager(organizer='event__organizer').__class__):
@@ -313,8 +353,8 @@ class ItemQuerySetManager(ScopedManager(organizer='event__organizer').__class__)
         super().__init__()
         self._queryset_class = ItemQuerySet
 
-    def filter_available(self, channel='web', voucher=None, allow_addons=False):
-        return filter_available(self.get_queryset(), channel, voucher, allow_addons)
+    def filter_available(self, channel='web', voucher=None, allow_addons=False, allow_cross_sell=False):
+        return filter_available(self.get_queryset(), channel, voucher, allow_addons, allow_cross_sell)
 
 
 class Item(LoggedModel):

src/pretix/base/services/cart.py

@@ -1542,10 +1542,9 @@ def add_items_to_cart(self, event: int, items: List[dict], cart_id: str=None, lo
 @app.task(base=ProfiledEventTask, bind=True, max_retries=5, default_retry_delay=1, throws=(CartError,))
 def apply_voucher(self, event: Event, voucher: str, cart_id: str=None, locale='en', sales_channel='web', override_now_dt: datetime=None) -> None:
     """
-    Removes a list of items from a user's cart.
     :param event: The event ID in question
     :param voucher: A voucher code
-    :param session: Session ID of a guest
+    :param cart_id: The cart ID of the cart to modify
     """
     with language(locale), time_machine_now_assigned(override_now_dt):
         try:
@@ -1566,10 +1565,10 @@ def apply_voucher(self, event: Event, voucher: str, cart_id: str=None, locale='e
 @app.task(base=ProfiledEventTask, bind=True, max_retries=5, default_retry_delay=1, throws=(CartError,))
 def remove_cart_position(self, event: Event, position: int, cart_id: str=None, locale='en', sales_channel='web', override_now_dt: datetime=None) -> None:
     """
-    Removes a list of items from a user's cart.
+    Removes an item specified by its position ID from a user's cart.
     :param event: The event ID in question
     :param position: A cart position ID
-    :param session: Session ID of a guest
+    :param cart_id: The cart ID of the cart to modify
     """
     with language(locale), time_machine_now_assigned(override_now_dt):
         try:
@@ -1590,9 +1589,9 @@ def remove_cart_position(self, event: Event, position: int, cart_id: str=None, l
 @app.task(base=ProfiledEventTask, bind=True, max_retries=5, default_retry_delay=1, throws=(CartError,))
 def clear_cart(self, event: Event, cart_id: str=None, locale='en', sales_channel='web', override_now_dt: datetime=None) -> None:
     """
-    Removes a list of items from a user's cart.
+    Removes all items from a user's cart.
     :param event: The event ID in question
-    :param session: Session ID of a guest
+    :param cart_id: The cart ID of the cart to modify
     """
     with language(locale), time_machine_now_assigned(override_now_dt):
         try:
@@ -1611,13 +1610,15 @@ def clear_cart(self, event: Event, cart_id: str=None, locale='en', sales_channel
 
 
 @app.task(base=ProfiledEventTask, bind=True, max_retries=5, default_retry_delay=1, throws=(CartError,))
-def set_cart_addons(self, event: Event, addons: List[dict], cart_id: str=None, locale='en',
+def set_cart_addons(self, event: Event, addons: List[dict], add_to_cart_items: List[dict], cart_id: str=None, locale='en',
                     invoice_address: int=None, sales_channel='web', override_now_dt: datetime=None) -> None:
     """
-    Removes a list of items from a user's cart.
+    Assigns addons to eligible products in a user's cart, adding and removing the addon products as necessary to
+    ensure the requested addon state.
     :param event: The event ID in question
     :param addons: A list of dicts with the keys addon_to, item, variation
-    :param session: Session ID of a guest
+    :param add_to_cart_items: A list of dicts with the keys item, variation, count, custom_price, voucher, seat ID
+    :param cart_id: The cart ID of the cart to modify
     """
     with language(locale), time_machine_now_assigned(override_now_dt):
         ia = False
@@ -1635,6 +1636,7 @@ def set_cart_addons(self, event: Event, addons: List[dict], cart_id: str=None, l
             try:
                 cm = CartManager(event=event, cart_id=cart_id, invoice_address=ia, sales_channel=sales_channel)
                 cm.set_addons(addons)
+                cm.add_new_items(add_to_cart_items)
                 cm.commit()
             except LockTimeoutException:
                 self.retry()

src/pretix/base/services/cross_selling.py

@@ -0,0 +1,232 @@
+#
+# This file is part of pretix (Community Edition).
+#
+# Copyright (C) 2014-2020 Raphael Michel and contributors
+# Copyright (C) 2020-2021 rami.io GmbH and contributors
+#
+# This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General
+# Public License as published by the Free Software Foundation in version 3 of the License.
+#
+# ADDITIONAL TERMS APPLY: Pursuant to Section 7 of the GNU Affero General Public License, additional terms are
+# applicable granting you additional permissions and placing additional restrictions on your usage of this software.
+# Please refer to the pretix LICENSE file to obtain the full terms applicable to this work. If you did not receive
+# this file, see <https://pretix.eu/about/en/license>.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
+# <https://www.gnu.org/licenses/>.
+#
+
+from collections import defaultdict
+from decimal import Decimal
+from itertools import groupby
+from math import inf
+from typing import List
+
+from django.utils.functional import cached_property
+
+from pretix.base.models import CartPosition, ItemCategory, SalesChannel
+from pretix.presale.views.event import get_grouped_items
+
+
+class DummyCategory:
+    """
+    Used to create fake category objects for displaying the same cross-selling category multiple times,
+    once for each subevent
+    """
+
+    def __init__(self, category: ItemCategory, subevent):
+        self.id = category.id
+        self.name = str(category.name)
+        self.subevent_name = str(subevent)
+        self.description = category.description
+
+
+class CrossSellingService:
+    def __init__(self, event, sales_channel: SalesChannel, cartpositions: List[CartPosition], customer):
+        self.event = event
+        self.sales_channel = sales_channel
+        self.cartpositions = cartpositions
+        self.customer = customer
+
+    def get_data(self):
+        if self.event.has_subevents:
+            subevents = set(pos.subevent for pos in self.cartpositions)
+            result = (
+                (DummyCategory(category, subevent),
+                 self._prepare_items(subevent, items_qs, discount_info),
+                 f'subevent_{subevent.pk}_')
+                for subevent in subevents
+                for (category, items_qs, discount_info) in self._applicable_categories(subevent.pk)
+            )
+        else:
+            result = (
+                (category,
+                 self._prepare_items(None, items_qs, discount_info),
+                 '')
+                for (category, items_qs, discount_info) in self._applicable_categories(0)
+            )
+        result = [(category, items, form_prefix) for (category, items, form_prefix) in result if len(items) > 0]
+        for category, items, form_prefix in result:
+            category.category_has_discount = any(item.original_price for item in items)
+        return result
+
+    def _applicable_categories(self, subevent_id):
+        return [
+            (c, products_qs, discount_info) for (c, products_qs, discount_info) in
+            (
+                (c, *self._get_visible_items_for_category(subevent_id, c))
+                for c in self.event.categories.filter(cross_selling_mode__isnull=False).prefetch_related('items')
+            )
+            if products_qs is not None
+        ]
+
+    def _get_visible_items_for_category(self, filter_subevent_id, category: ItemCategory):
+        """
+        If this category should be visible in the cross-selling step for a given cart and sales_channel, this method
+        returns a queryset of the items that should be displayed, as well as a dict giving additional information on them.
+
+        :returns: (QuerySet<Item>, dict<(subevent_id, item_pk): (max_count, discount_rule)>)
+            max_count is `inf` if the item should not be limited
+            discount_rule is None if the item will not be discounted
+        """
+        if category.cross_selling_mode is None:
+            return None, {}
+        if category.cross_selling_condition == 'always':
+            return category.items.all(), {}
+        if category.cross_selling_condition == 'products':
+            match = set(match.pk for match in category.cross_selling_match_products.only('pk'))  # TODO prefetch this
+            return (category.items.all(), {}) if any(pos.item.pk in match for pos in self.cartpositions) else (None, {})
+        if category.cross_selling_condition == 'discounts':
+            my_item_pks = [item.id for item in category.items.all()]
+            potential_discount_items = {
+                item.pk: (max_count, discount_rule)
+                for subevent_id, item, max_count, discount_rule in self._potential_discounts_by_subevent_and_item_for_current_cart
+                if max_count > 0 and item.pk in my_item_pks and item.is_available() and (subevent_id == filter_subevent_id or subevent_id is None)
+            }
+            return category.items.filter(pk__in=potential_discount_items), potential_discount_items
+
+    @cached_property
+    def _potential_discounts_by_subevent_and_item_for_current_cart(self):
+        potential_discounts_by_cartpos = defaultdict(list)
+
+        from ..services.pricing import apply_discounts
+        self._discounted_prices = apply_discounts(
+            self.event,
+            self.sales_channel,
+            [
+                (cp.item_id, cp.subevent_id, cp.line_price_gross, bool(cp.addon_to), cp.is_bundled,
+                 cp.listed_price - cp.price_after_voucher)
+                for cp in self.cartpositions
+            ],
+            collect_potential_discounts=potential_discounts_by_cartpos
+        )
+
+        # flatten potential_discounts_by_cartpos (a dict of lists of potential discounts) into a set of potential discounts
+        # (which is technically stored as a dict, but we use it as an OrderedSet here)
+        potential_discount_set = dict.fromkeys(
+            info for lst in potential_discounts_by_cartpos.values() for info in lst)
+
+        # sum up the max_counts and pass them on (also pass on the discount_rules so we can calculate actual discounted prices later):
+        # group by benefit product
+        # - max_count for product: sum up max_counts
+        # - discount_rule for product: take first discount_rule
+
+        def discount_info(subevent_id, item, infos_for_item):
+            infos_for_item = list(infos_for_item)
+            return (
+                subevent_id,
+                item,
+                sum(max_count for (subevent_id, item, discount_rule, max_count, i) in infos_for_item),
+                next(discount_rule for (subevent_id, item, discount_rule, max_count, i) in infos_for_item),
+            )
+
+        return [
+            discount_info(subevent_id, item, infos_for_item) for (subevent_id, item), infos_for_item in
+            groupby(
+                sorted(
+                    (
+                        (subevent_id, item, discount_rule, max_count, i)
+                        for (discount_rule, max_count, i, subevent_id) in potential_discount_set.keys()
+                        for item in discount_rule.benefit_limit_products.all()
+                    ),
+                    key=lambda tup: (tup[0], tup[1].pk)
+                ),
+                lambda tup: (tup[0], tup[1]))
+        ]
+
+    def _prepare_items(self, subevent, items_qs, discount_info):
+        items, _btn = get_grouped_items(
+            self.event,
+            subevent=subevent,
+            voucher=None,
+            channel=self.sales_channel,
+            base_qs=items_qs,
+            allow_addons=False,
+            allow_cross_sell=True,
+            memberships=(
+                self.customer.usable_memberships(
+                    for_event=subevent or self.event,
+                    testmode=self.event.testmode
+                )
+                if self.customer else None
+            ),
+        )
+        new_items = list()
+        for item in items:
+            max_count = inf
+            if item.pk in discount_info:
+                (max_count, discount_rule) = discount_info[item.pk]
+
+                # only benefit_only_apply_to_cheapest_n_matches discounted items have a max_count, all others get 'inf'
+                if not max_count:
+                    max_count = inf
+
+                # calculate discounted price
+                if discount_rule and discount_rule.benefit_discount_matching_percent > 0:
+                    if not item.has_variations:
+                        item.original_price = item.original_price or item.display_price
+                        previous_price = item.display_price
+                        new_price = (
+                            previous_price * (
+                                (Decimal('100.00') - discount_rule.benefit_discount_matching_percent) / Decimal('100.00'))
+                        )
+                        item.display_price = new_price
+                    else:
+                        # discounts always match "whole" items, not specific variations -> we apply the discount to all
+                        # available variations of the item
+                        for var in item.available_variations:
+                            var.original_price = var.original_price or var.display_price
+                            previous_price = var.display_price
+                            new_price = (
+                                previous_price * (
+                                    (Decimal('100.00') - discount_rule.benefit_discount_matching_percent) / Decimal('100.00'))
+                            )
+                            var.display_price = new_price
+
+            if not item.has_variations:
+                # reduce order_max by number of items already in cart (prevent recommending a product the user can't add anyway)
+                item.order_max = min(
+                    item.order_max - sum(1 for pos in self.cartpositions if pos.item_id == item.pk),
+                    max_count
+                )
+                if item.order_max > 0:
+                    new_items.append(item)
+            else:
+                new_vars = list()
+                for var in item.available_variations:
+                    # reduce order_max by number of items already in cart (prevent recommending a product the user can't add anyway)
+                    var.order_max = min(
+                        var.order_max - sum(1 for pos in self.cartpositions if pos.item_id == item.pk and pos.variation_id == var.pk),
+                        max_count
+                    )
+                    if var.order_max > 0:
+                        new_vars.append(var)
+                if len(new_vars):
+                    item.available_variations = new_vars
+                    new_items.append(item)
+
+        return new_items

src/pretix/base/services/pricing.py

@@ -20,8 +20,9 @@
 # <https://www.gnu.org/licenses/>.
 #
 import re
+from collections import defaultdict
 from decimal import Decimal
-from typing import List, Optional, Tuple
+from typing import List, Optional, Tuple, Union
 
 from django import forms
 from django.db.models import Q
@@ -31,6 +32,7 @@ from pretix.base.models import (
     AbstractPosition, InvoiceAddress, Item, ItemAddOn, ItemVariation,
     SalesChannel, Voucher,
 )
+from pretix.base.models.discount import Discount, PositionInfo
 from pretix.base.models.event import Event, SubEvent
 from pretix.base.models.tax import TAXED_ZERO, TaxedPrice, TaxRule
 from pretix.base.timemachine import time_machine_now
@@ -155,14 +157,22 @@ def get_line_price(price_after_voucher: Decimal, custom_price_input: Decimal, cu
     return price
 
 
-def apply_discounts(event: Event, sales_channel: str,
-                    positions: List[Tuple[int, Optional[int], Decimal, bool, bool]]) -> List[Decimal]:
+def apply_discounts(event: Event, sales_channel: Union[str, SalesChannel],
+                    positions: List[Tuple[int, Optional[int], Decimal, bool, bool, Decimal]],
+                    collect_potential_discounts: Optional[defaultdict]=None) -> List[Tuple[Decimal, Optional[Discount]]]:
     """
     Applies any dynamic discounts to a cart
 
     :param event: Event the cart belongs to
     :param sales_channel: Sales channel the cart was created with
     :param positions: Tuple of the form ``(item_id, subevent_id, line_price_gross, is_addon_to, is_bundled, voucher_discount)``
+    :param collect_potential_discounts: If a `defaultdict(list)` is supplied, all discounts that could be applied to the cart
+    based on the "consumed" items, but lack matching "benefitting" items will be collected therein.
+    The dict will contain a mapping from index in the `positions` list of the item that could be consumed, to a list
+    of tuples describing the discounts that could be applied in the form `(discount, max_count, grouping_id)`.
+    `max_count` is either the maximum number of benefitting items that the discount would apply to, or `inf` if that number
+    is not limited. The `grouping_id` can be used to distinguish several occurrences of the same discount.
+
     :return: A list of ``(new_gross_price, discount)`` tuples in the same order as the input
     """
     if isinstance(sales_channel, SalesChannel):
@@ -177,10 +187,10 @@ def apply_discounts(event: Event, sales_channel: str,
     ).prefetch_related('condition_limit_products', 'benefit_limit_products').order_by('position', 'pk')
     for discount in discount_qs:
         result = discount.apply({
-            idx: (item_id, subevent_id, line_price_gross, is_addon_to, voucher_discount)
+            idx: PositionInfo(item_id, subevent_id, line_price_gross, is_addon_to, voucher_discount)
             for idx, (item_id, subevent_id, line_price_gross, is_addon_to, is_bundled, voucher_discount) in enumerate(positions)
             if not is_bundled and idx not in new_prices
-        })
+        }, collect_potential_discounts)
         for k in result.keys():
             result[k] = (result[k], discount)
         new_prices.update(result)

src/pretix/control/forms/item.py

@@ -40,7 +40,8 @@ from urllib.parse import urlencode
 from django import forms
 from django.conf import settings
 from django.core.exceptions import ValidationError
-from django.db.models import Max
+from django.db.models import Max, Q
+from django.forms import ChoiceField, RadioSelect
 from django.forms.formsets import DELETION_FIELD_NAME
 from django.urls import reverse
 from django.utils.functional import cached_property
@@ -79,11 +80,67 @@ class CategoryForm(I18nModelForm):
             'name',
             'internal_name',
             'description',
-            'is_addon'
+            'cross_selling_condition',
+            'cross_selling_match_products',
         ]
         widgets = {
             'description': I18nMarkdownTextarea,
+            'cross_selling_condition': RadioSelect,
         }
+        field_classes = {
+            'cross_selling_match_products': SafeModelMultipleChoiceField,
+        }
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        tpl = '{} &nbsp; <span class="text-muted">{}</span>'
+        self.fields['category_type'] = ChoiceField(widget=RadioSelect, choices=(
+            ('normal', mark_safe(tpl.format(
+                _('Normal category'),
+                _('Products in this category are regular products displayed on the front page.')
+            )),),
+            ('addon', mark_safe(tpl.format(
+                _('Add-on product category'),
+                _('Products in this category are add-on products and can only be bought as add-ons.')
+            )),),
+            ('only', mark_safe(tpl.format(
+                _('Cross-selling category'),
+                _('Products in this category are regular products, but are only shown in the cross-selling step, '
+                  'according to the configuration below.')
+            )),),
+            ('both', mark_safe(tpl.format(
+                _('Normal + cross-selling category'),
+                _('Products in this category are regular products displayed on the front page, but are additionally '
+                  'shown in the cross-selling step, according to the configuration below.')
+            )),),
+        ))
+        self.fields['category_type'].initial = self.instance.category_type
+
+        self.fields['cross_selling_condition'].widget.attrs['data-display-dependency'] = '#id_category_type_2,#id_category_type_3'
+        self.fields['cross_selling_condition'].widget.attrs['data-disable-dependent'] = 'true'
+        self.fields['cross_selling_condition'].widget.choices = self.fields['cross_selling_condition'].widget.choices[1:]
+        self.fields['cross_selling_condition'].required = False
+
+        self.fields['cross_selling_match_products'].widget = forms.CheckboxSelectMultiple(
+            attrs={
+                'class': 'scrolling-multiple-choice',
+                'data-display-dependency': '#id_cross_selling_condition_2'
+            }
+        )
+        self.fields['cross_selling_match_products'].queryset = self.event.items.filter(
+            # don't show products which are only visible in addon/cross-sell step themselves
+            Q(category__isnull=True) | Q(
+                Q(category__is_addon=False) & Q(Q(category__cross_selling_mode='both') | Q(category__cross_selling_mode__isnull=True))
+            )
+        )
+
+    def clean(self):
+        d = super().clean()
+        if d.get('category_type') == 'only' or d.get('category_type') == 'both':
+            if not d.get('cross_selling_condition'):
+                raise ValidationError({'cross_selling_condition': [_('This field is required')]})
+        self.instance.category_type = d.get('category_type')
+        return d
 
 
 class QuestionForm(I18nModelForm):

src/pretix/control/templates/pretixcontrol/items/categories.html

@@ -31,6 +31,7 @@
                 <thead>
                 <tr>
                     <th>{% trans "Product categories" %}</th>
+                    <th>{% trans "Category type" %}</th>
                     <th class="action-col-2"></th>
                 </tr>
                 </thead>
@@ -40,6 +41,9 @@
                         <td>
                             <strong><a href="{% url "control:event.items.categories.edit" organizer=request.event.organizer.slug event=request.event.slug category=c.id %}">{{ c.internal_name|default:c.name }}</a></strong>
                         </td>
+                        <td>
+                            {{ c.get_category_type_display }}
+                        </td>
                         <td class="text-right flip">
                             <button title="{% trans "Move up" %}" formaction="{% url "control:event.items.categories.up" organizer=request.event.organizer.slug event=request.event.slug category=c.id %}" class="btn btn-default btn-sm sortable-up"{% if forloop.counter0 == 0 and not page_obj.has_previous %} disabled{% endif %}><i class="fa fa-arrow-up"></i></button>
                             <button title="{% trans "Move down" %}" formaction="{% url "control:event.items.categories.down" organizer=request.event.organizer.slug event=request.event.slug category=c.id %}" class="btn btn-default btn-sm sortable-down"{% if forloop.revcounter0 == 0 and not page_obj.has_next %} disabled{% endif %}><i class="fa fa-arrow-down"></i></button>

src/pretix/control/templates/pretixcontrol/items/category.html

@@ -16,7 +16,9 @@
                         {% bootstrap_field form.internal_name layout="control" %}
                     </div>
                     {% bootstrap_field form.description layout="control" %}
-                    {% bootstrap_field form.is_addon layout="control" %}
+                    {% bootstrap_field form.category_type layout="control" horizontal_field_class="big-radio-wrapper col-lg-9" %}
+                    {% bootstrap_field form.cross_selling_condition layout="control" horizontal_field_class="col-lg-9" %}
+                    {% bootstrap_field form.cross_selling_match_products layout="control" %}
                 </fieldset>
             </div>
             {% if category %}

src/pretix/presale/checkoutflow.py

@@ -65,6 +65,7 @@ from pretix.base.services.cart import (
     CartError, CartManager, add_payment_to_cart, error_messages, get_fees,
     set_cart_addons,
 )
+from pretix.base.services.cross_selling import CrossSellingService
 from pretix.base.services.memberships import validate_memberships_in_order
 from pretix.base.services.orders import perform_order
 from pretix.base.services.tasks import EventTask
@@ -93,7 +94,8 @@ from pretix.presale.views import (
     CartMixin, get_cart, get_cart_is_free, get_cart_total,
 )
 from pretix.presale.views.cart import (
-    cart_session, create_empty_cart_id, get_or_create_cart_id,
+    _items_from_post_data, cart_session, create_empty_cart_id,
+    get_or_create_cart_id,
 )
 from pretix.presale.views.event import get_grouped_items
 from pretix.presale.views.questions import QuestionsViewMixin
@@ -486,9 +488,31 @@ class AddOnsStep(CartMixin, AsyncAction, TemplateFlowStep):
     label = pgettext_lazy('checkoutflow', 'Add-on products')
     icon = 'puzzle-piece'
 
+    def _check_is_applicable(self, request):
+        self.request = request
+
+        # check whether addons are applicable
+        if get_cart(request).filter(item__addons__isnull=False).exists():
+            return True
+
+        # don't re-check whether cross-selling is applicable if we're already past the AddOnsStep
+        cur_step_identifier = request.resolver_match.kwargs.get('step')
+        is_past_this_step = any(step.identifier == cur_step_identifier for step in request._checkout_flow[request._checkout_flow.index(self) + 1:])
+        if is_past_this_step:
+            applicable = self.cart_session.get('_checkoutflow_addons_applicable', None)
+            if applicable is not None:
+                return applicable
+
+        # check whether cross-selling is applicable
+        applicable = self.cross_selling_is_applicable
+        self.cart_session['_checkoutflow_addons_applicable'] = applicable
+        return applicable
+
     def is_applicable(self, request):
         if not hasattr(request, '_checkoutflow_addons_applicable'):
-            request._checkoutflow_addons_applicable = get_cart(request).filter(item__addons__isnull=False).exists()
+            cur_step_identifier = request.resolver_match.kwargs.get('step')
+            request._checkoutflow_addons_applicable = self._check_is_applicable(request) or cur_step_identifier == self.identifier
+
         return request._checkoutflow_addons_applicable
 
     def is_completed(self, request, warn=False):
@@ -605,10 +629,21 @@ class AddOnsStep(CartMixin, AsyncAction, TemplateFlowStep):
                 formset.append(formsetentry)
         return formset
 
+    @cached_property
+    def cross_selling_is_applicable(self):
+        return any(len(items) > 0 for (category, items, form_prefix) in self.cross_selling_data)
+
+    @cached_property
+    def cross_selling_data(self):
+        return CrossSellingService(
+            self.request.event, self.request.sales_channel, self.positions, self.request.customer
+        ).get_data()
+
     def get_context_data(self, **kwargs):
         ctx = super().get_context_data(**kwargs)
         ctx['forms'] = self.forms
         ctx['cart'] = self.get_cart()
+        ctx['cross_selling_data'] = self.cross_selling_data
         return ctx
 
     def get_success_message(self, value):
@@ -687,7 +722,7 @@ class AddOnsStep(CartMixin, AsyncAction, TemplateFlowStep):
 
     def post(self, request, *args, **kwargs):
         self.request = request
-        data = []
+        addons = []
         for f in self.forms:
             for c in f['categories']:
                 try:
@@ -697,7 +732,7 @@ class AddOnsStep(CartMixin, AsyncAction, TemplateFlowStep):
                     return self.get(request, *args, **kwargs)
 
                 for (i, v), (c, price) in selected.items():
-                    data.append({
+                    addons.append({
                         'addon_to': f['pos'].pk,
                         'item': i.pk,
                         'variation': v.pk if v else None,
@@ -705,7 +740,9 @@ class AddOnsStep(CartMixin, AsyncAction, TemplateFlowStep):
                         'price': price,
                     })
 
-        return self.do(self.request.event.id, data, get_or_create_cart_id(self.request),
+        add_to_cart_items = _items_from_post_data(self.request, warn_if_empty=False)
+
+        return self.do(self.request.event.id, addons, add_to_cart_items, get_or_create_cart_id(self.request),
                        invoice_address=self.invoice_address.pk, locale=get_language(),
                        sales_channel=request.sales_channel.identifier, override_now_dt=time_machine_now(default=None))
 

src/pretix/presale/templates/pretixpresale/event/checkout_addons.html

@@ -6,9 +6,12 @@
 {% load money %}
 {% load thumb %}
 {% block inner %}
-    <p>
-        {% trans "For some of the products in your cart, you can choose additional options before you continue." %}
-    </p>
+
+    {% if forms %}
+        <p>
+            {% trans "For some of the products in your cart, you can choose additional options before you continue." %}
+        </p>
+    {% endif %}
     <form class="form-horizontal" method="post" data-asynctask
             data-asynctask-headline="{% trans "We're now trying to book these add-ons for you!" %}">
         {% csrf_token %}
@@ -17,10 +20,12 @@
                 <details class="panel panel-default" open>
                     <summary class="panel-heading">
                         <h3 class="panel-title">
-                            <span class="sr-only">{% trans "Add-ons:" %}</span>
-                            <strong>{{ form.item.name }}{% if form.variation %}
-                                – {{ form.variation }}
-                            {% endif %}</strong>
+                            <span>
+                                {% trans "Additional options for" %}
+                                <strong>{{ form.item.name }}{% if form.variation %}
+                                    – {{ form.variation }}
+                                {% endif %}</strong>
+                            </span>
                         </h3>
                     </summary>
                     <div id="cp{{ form.pos.pk }}">
@@ -46,6 +51,20 @@
                 </details>
             {% endfor %}
         </div>
+
+        {% if cross_selling_data %}
+            <details class="panel panel-default cross-selling" open>
+                <summary class="panel-heading">
+                    <h3 class="panel-title">
+                        {% trans "More recommendations" %}
+                    </h3>
+                </summary>
+                <div class="panel-body">
+                    {% include "pretixpresale/event/fragment_product_list.html" with items_by_category=cross_selling_data ev=event %}
+                </div>
+            </details>
+        {% endif %}
+
         <div class="row checkout-button-row">
             <div class="col-md-4 col-sm-6">
                 <a class="btn btn-block btn-default btn-lg"

src/pretix/presale/templates/pretixpresale/event/fragment_product_list.html

@@ -5,20 +5,31 @@
 {% load thumb %}
 {% load eventsignal %}
 {% load rich_text %}
-{% for tup in items_by_category %}
-    {% if tup.0 %}
-        <section aria-labelledby="category-{{ tup.0.id }}"{% if tup.0.description %} aria-describedby="category-info-{{ tup.0.id }}"{% endif %}>
-            <h3 id="category-{{ tup.0.id }}">{{ tup.0.name }}</h3>
-            {% if tup.0.description %}
-                <div id="category-info-{{ tup.0.id }}">{{ tup.0.description|localize|rich_text }}</div>
+{% for tup in items_by_category %}{% with category=tup.0 items=tup.1 form_prefix=tup.2 %}
+    {% if category %}
+        <section aria-labelledby="{{ form_prefix }}category-{{ category.id }}"{% if category.description %} aria-describedby="{{ form_prefix }}category-info-{{ category.id }}"{% endif %}>
+            <h3 id="{{ form_prefix }}category-{{ category.id }}">{{ category.name }}
+                {% if category.subevent_name %}
+                    <small class="text-muted"><i class="fa fa-calendar"></i> {{ category.subevent_name }}</small>
+                {% endif %}
+                {% if category.category_has_discount %}
+                    <small class="text-success">
+                        <i class="fa fa-star" aria-hidden="true"></i>
+                        <span class="sr-only">Congratulations!</span>
+                        {% trans "Your order qualifies for a discount" %}
+                    </small>
+                {% endif %}
+            </h3>
+            {% if category.description %}
+                <div id="{{ form_prefix }}category-info-{{ category.id }}">{{ category.description|localize|rich_text }}</div>
             {% endif %}
     {% else %}
-        <section aria-labelledby="category-none">
-            <h3 id="category-none" class="sr-only">{% trans "Uncategorized items" %}</h3>
+        <section aria-labelledby="{{ form_prefix }}category-none">
+            <h3 id="{{ form_prefix }}category-none" class="sr-only">{% trans "Uncategorized items" %}</h3>
     {% endif %}
-        {% for item in tup.1 %}
+        {% for item in items %}
             {% if item.has_variations %}
-                <article aria-labelledby="item-{{ item.pk }}-legend"{% if item.description %} aria-describedby="item-{{ item.pk }}-description"{% endif %} class="item-with-variations{% if event.settings.show_variations_expanded %} details-open{% endif %}" id="item-{{ item.pk }}">
+                <article aria-labelledby="{{ form_prefix }}item-{{ item.pk }}-legend"{% if item.description %} aria-describedby="{{ form_prefix }}item-{{ item.pk }}-description"{% endif %} class="item-with-variations{% if event.settings.show_variations_expanded %} details-open{% endif %}" id="{{ form_prefix }}item-{{ item.pk }}">
                     <div class="row product-row headline">
                         <div class="col-md-8 col-sm-6 col-xs-12">
                             {% if item.picture %}
@@ -32,9 +43,9 @@
                                 </a>
                             {% endif %}
                             <div class="product-description {% if item.picture %}with-picture{% endif %}">
-                                <h4 id="item-{{ item.pk }}-legend">{{ item.name }}</h4>
+                                <h4 id="{{ form_prefix }}item-{{ item.pk }}-legend">{{ item.name }}</h4>
                                 {% if item.description %}
-                                    <div id="item-{{ item.pk }}-description" class="product-description">
+                                    <div id="{{ form_prefix }}item-{{ item.pk }}-description" class="product-description">
                                         {{ item.description|localize|rich_text }}
                                     </div>
                                 {% endif %}
@@ -101,14 +112,14 @@
                     </div>
                     <div class="variations {% if not event.settings.show_variations_expanded %}variations-collapsed{% endif %}">
                         {% for var in item.available_variations %}
-                            <article aria-labelledby="item-{{ item.pk }}-{{ var.pk }}-legend"{% if var.description %} aria-describedby="item-{{ item.pk }}-{{ var.pk }}-description"{% endif %} class="row product-row variation" id="item-{{ item.pk }}-{{ var.pk }}"
+                            <article aria-labelledby="{{ form_prefix }}item-{{ item.pk }}-{{ var.pk }}-legend"{% if var.description %} aria-describedby="{{ form_prefix }}item-{{ item.pk }}-{{ var.pk }}-description"{% endif %} class="row product-row variation" id="{{ form_prefix }}item-{{ item.pk }}-{{ var.pk }}"
                             {% if not item.free_price %}
                                 data-price="{% if event.settings.display_net_prices %}{{ var.display_price.net|unlocalize }}{% else %}{{ var.display_price.gross|unlocalize }}{% endif %}"
                             {% endif %}>
                                 <div class="col-md-8 col-sm-6 col-xs-12">
-                                    <h5 id="item-{{ item.pk }}-{{ var.pk }}-legend">{{ var }}</h5>
+                                    <h5 id="{{ form_prefix }}item-{{ item.pk }}-{{ var.pk }}-legend">{{ var }}</h5>
                                     {% if var.description %}
-                                        <div id="item-{{ item.pk }}-{{ var.pk }}-description" class="variation-description">
+                                        <div id="{{ form_prefix }}item-{{ item.pk }}-{{ var.pk }}-description" class="variation-description">
                                             {{ var.description|localize|rich_text }}
                                         </div>
                                     {% endif %}
@@ -139,11 +150,11 @@
                                         <div class="input-group input-group-price">
                                             <span class="input-group-addon">{{ event.currency }}</span>
                                             <input type="number" class="form-control input-item-price"
-                                                   id="price-variation-{{ item.pk }}-{{ var.pk }}"
+                                                   id="{{ form_prefix }}price-variation-{{ item.pk }}-{{ var.pk }}"
                                                    {% if not ev.presale_is_running %}disabled{% endif %}
                                                    placeholder="0"
                                                    min="{% if event.settings.display_net_prices %}{{ var.display_price.net|money_numberfield:event.currency }}{% else %}{{ var.display_price.gross|money_numberfield:event.currency }}{% endif %}"
-                                                   name="price_{{ item.id }}_{{ var.id }}"
+                                                   name="{{ form_prefix }}price_{{ item.id }}_{{ var.id }}"
                                                    {% if var.suggested_price.gross != var.display_price.gross %}
                                                        {% if event.settings.display_net_prices %}
                                                            title="{% blocktrans trimmed with item=var.value price=var.display_price.net|money:event.currency %}Modify price for {{ item }}, at least {{ price }}{% endblocktrans %}"
@@ -200,16 +211,16 @@
                                                        data-checked-onchange="price-variation-{{ item.pk }}-{{ var.pk }}"
                                                     {% endif %}
                                                        {% if not ev.presale_is_running %}disabled{% endif %}
-                                                       id="variation_{{ item.id }}_{{ var.id }}"
-                                                       name="variation_{{ item.id }}_{{ var.id }}"
+                                                       id="{{ form_prefix }}variation_{{ item.id }}_{{ var.id }}"
+                                                       name="{{ form_prefix }}variation_{{ item.id }}_{{ var.id }}"
                                                        aria-label="{% blocktrans with item=item.name var=var %}Add {{ item }}, {{ var }} to cart{% endblocktrans %}"
-                                                       {% if var.description %} aria-describedby="item-{{ item.pk }}-{{ var.pk }}-description"{% endif %}>
+                                                       {% if var.description %} aria-describedby="{{ form_prefix }}item-{{ item.pk }}-{{ var.pk }}-description"{% endif %}>
                                                 <i class="fa fa-shopping-cart" aria-hidden="true"></i>
                                                 {% trans "Select" context "checkbox" %}
                                             </label>
                                         {% else %}
                                             <div class="input-item-count-group">
-                                                <button type="button" data-step="-1" data-controls="variation_{{ item.id }}_{{ var.id }}" class="btn btn-default input-item-count-dec" aria-label="{% trans "Decrease quantity" %}"
+                                                <button type="button" data-step="-1" data-controls="{{ form_prefix }}variation_{{ item.id }}_{{ var.id }}" class="btn btn-default input-item-count-dec" aria-label="{% trans "Decrease quantity" %}"
                                                     {% if not ev.presale_is_running %}disabled{% endif %}>-</button>
                                                 <input type="number" class="form-control input-item-count" placeholder="0" min="0"
                                                        {% if not ev.presale_is_running %}disabled{% endif %}
@@ -217,10 +228,10 @@
                                                            data-checked-onchange="price-variation-{{ item.pk }}-{{ var.pk }}"
                                                         {% endif %}
                                                        max="{{ var.order_max }}"
-                                                       id="variation_{{ item.id }}_{{ var.id }}"
-                                                       name="variation_{{ item.id }}_{{ var.id }}"
+                                                       id="{{ form_prefix }}variation_{{ item.id }}_{{ var.id }}"
+                                                       name="{{ form_prefix }}variation_{{ item.id }}_{{ var.id }}"
                                                        aria-label="{% blocktrans with item=item.name var=var.name %}Quantity of {{ item }}, {{ var }} to order{% endblocktrans %}">
-                                                <button type="button" data-step="1" data-controls="variation_{{ item.id }}_{{ var.id }}" class="btn btn-default input-item-count-inc" aria-label="{% trans "Increase quantity" %}"
+                                                <button type="button" data-step="1" data-controls="{{ form_prefix }}variation_{{ item.id }}_{{ var.id }}" class="btn btn-default input-item-count-inc" aria-label="{% trans "Increase quantity" %}"
                                                     {% if not ev.presale_is_running %}disabled{% endif %}>+</button>
                                             </div>
                                         {% endif %}
@@ -234,7 +245,7 @@
                     </div>
                 </article>
             {% else %}
-                <article aria-labelledby="item-{{ item.pk }}-legend"{% if item.description %} aria-describedby="item-{{ item.pk }}-description"{% endif %} class="row product-row simple" id="item-{{ item.pk }}"
+                <article aria-labelledby="{{ form_prefix }}item-{{ item.pk }}-legend"{% if item.description %} aria-describedby="{{ form_prefix }}item-{{ item.pk }}-description"{% endif %} class="row product-row simple" id="{{ form_prefix }}item-{{ item.pk }}"
                 {% if not item.free_price %}
                     data-price="{% if event.settings.display_net_prices %}{{ item.display_price.net|unlocalize }}{% else %}{{ item.display_price.gross|unlocalize }}{% endif %}"
                 {% endif %}>
@@ -250,9 +261,9 @@
                             </a>
                         {% endif %}
                         <div class="product-description {% if item.picture %}with-picture{% endif %}">
-                            <h4 id="item-{{ item.pk }}-legend">{{ item.name }}</h4>
+                            <h4 id="{{ form_prefix }}item-{{ item.pk }}-legend">{{ item.name }}</h4>
                             {% if item.description %}
-                                <div id="item-{{ item.pk }}-description" class="product-description">
+                                <div id="{{ form_prefix }}item-{{ item.pk }}-description" class="product-description">
                                     {{ item.description|localize|rich_text }}
                                 </div>
                             {% endif %}
@@ -293,10 +304,10 @@
                                 <label class="sr-only" for="price-item-{{ item.pk }}">{% blocktrans trimmed with item=item.name currency=event.currency %}Set price in {{ currency }} for {{ item }}{% endblocktrans %}</label>
                                 <span class="input-group-addon" aria-hidden="true">{{ event.currency }}</span>
                                 <input type="number" class="form-control input-item-price" placeholder="0"
-                                       id="price-item-{{ item.pk }}"
+                                       id="{{ form_prefix }}price-item-{{ item.pk }}"
                                        {% if not ev.presale_is_running %}disabled{% endif %}
                                        min="{% if event.settings.display_net_prices %}{{ item.display_price.net|money_numberfield:event.currency }}{% else %}{{ item.display_price.gross|money_numberfield:event.currency }}{% endif %}"
-                                       name="price_{{ item.id }}"
+                                       name="{{ form_prefix }}price_{{ item.id }}"
                                        {% if item.suggested_price.gross != item.display_price.gross %}
                                            {% if event.settings.display_net_prices %}
                                                 title="{% blocktrans trimmed with item=item.name price=item.display_price.net|money:event.currency %}Modify price for {{ item }}, at least {{ price }}{% endblocktrans %}"
@@ -352,15 +363,15 @@
                                            data-checked-onchange="price-item-{{ item.pk }}"
                                         {% endif %}
                                            {% if not ev.presale_is_running %}disabled{% endif %}
-                                           name="item_{{ item.id }}" id="item_{{ item.id }}"
+                                           name="{{ form_prefix }}item_{{ item.id }}" id="{{ form_prefix }}item_{{ item.id }}"
                                            aria-label="{% blocktrans with item=item.name %}Add {{ item }} to cart{% endblocktrans %}"
-                                           {% if item.description %} aria-describedby="item-{{ item.id }}-description"{% endif %}>
+                                           {% if item.description %} aria-describedby="{{ form_prefix }}item-{{ item.id }}-description"{% endif %}>
                                         <i class="fa fa-shopping-cart" aria-hidden="true"></i>
                                         {% trans "Select" context "checkbox" %}
                                 </label>
                             {% else %}
                                 <div class="input-item-count-group">
-                                    <button type="button" data-step="-1" data-controls="item_{{ item.id }}" class="btn btn-default input-item-count-dec" aria-label="{% trans "Decrease quantity" %}"
+                                    <button type="button" data-step="-1" data-controls="{{ form_prefix }}item_{{ item.id }}" class="btn btn-default input-item-count-dec" aria-label="{% trans "Decrease quantity" %}"
                                         {% if not ev.presale_is_running %}disabled{% endif %}>-</button>
                                     <input type="number" class="form-control input-item-count" placeholder="0" min="0"
                                            {% if not ev.presale_is_running %}disabled{% endif %}
@@ -369,11 +380,11 @@
                                                data-checked-onchange="price-item-{{ item.pk }}"
                                             {% endif %}
                                            max="{{ item.order_max }}"
-                                           name="item_{{ item.id }}"
-                                           id="item_{{ item.id }}"
+                                           name="{{ form_prefix }}item_{{ item.id }}"
+                                           id="{{ form_prefix }}item_{{ item.id }}"
                                            aria-label="{% blocktrans with item=item.name %}Quantity of {{ item }} to order{% endblocktrans %}"
-                                           {% if item.description %} aria-describedby="item-{{ item.id }}-description"{% endif %}>
-                                    <button type="button" data-step="1" data-controls="item_{{ item.id }}" class="btn btn-default input-item-count-inc" aria-label="{% trans "Increase quantity" %}"
+                                           {% if item.description %} aria-describedby="{{ form_prefix }}item-{{ item.id }}-description"{% endif %}>
+                                    <button type="button" data-step="1" data-controls="{{ form_prefix }}item_{{ item.id }}" class="btn btn-default input-item-count-inc" aria-label="{% trans "Increase quantity" %}"
                                         {% if not ev.presale_is_running %}disabled{% endif %}>+</button>
                                 </div>
                             {% endif %}
@@ -386,4 +397,4 @@
             {% endif %}
         {% endfor %}
     </section>
-{% endfor %}
+{% endwith %}{% endfor %}

src/pretix/presale/views/__init__.py

@@ -318,14 +318,14 @@ def cart_exists(request):
 
 def get_cart(request):
     from pretix.presale.views.cart import get_or_create_cart_id
-    qqs = request.event.questions.all()
-    qqs = qqs.filter(ask_during_checkin=False, hidden=False)
 
     if not hasattr(request, '_cart_cache'):
         cart_id = get_or_create_cart_id(request, create=False)
         if not cart_id:
             request._cart_cache = CartPosition.objects.none()
         else:
+            qqs = request.event.questions.all()
+            qqs = qqs.filter(ask_during_checkin=False, hidden=False)
             request._cart_cache = CartPosition.objects.filter(
                 cart_id=cart_id, event=request.event
             ).annotate(

src/pretix/presale/views/cart.py

@@ -151,104 +151,114 @@ class CartActionMixin:
         except InvoiceAddress.DoesNotExist:
             return InvoiceAddress()
 
-    def _item_from_post_value(self, key, value, voucher=None, voucher_ignore_if_redeemed=False):
-        if value.strip() == '' or '_' not in key:
-            return
 
-        if not key.startswith('item_') and not key.startswith('variation_') and not key.startswith('seat_'):
-            return
-
-        parts = key.split("_")
-        price = self.request.POST.get('price_' + "_".join(parts[1:]), "")
-        subevent = None
-        if 'subevent' in self.request.POST:
-            try:
-                subevent = int(self.request.POST.get('subevent'))
-            except ValueError:
-                pass
-
-        if key.startswith('seat_'):
-            try:
-                return {
-                    'item': int(parts[1]),
-                    'variation': int(parts[2]) if len(parts) > 2 else None,
-                    'count': 1,
-                    'seat': value,
-                    'price': price,
-                    'voucher': voucher,
-                    'voucher_ignore_if_redeemed': voucher_ignore_if_redeemed,
-                    'subevent': subevent
-                }
-            except ValueError:
-                raise CartError(_('Please enter numbers only.'))
+def _item_from_post_value(request, key, value, voucher=None, voucher_ignore_if_redeemed=False):
+    if value.strip() == '' or '_' not in key:
+        return
 
+    subevent = None
+    if key.startswith('subevent_'):
         try:
-            amount = int(value)
+            parts = key.split('_', 2)
+            subevent = int(parts[1])
+            key = parts[2]
+        except ValueError:
+            pass
+    elif 'subevent' in request.POST:
+        try:
+            subevent = int(request.POST.get('subevent'))
         except ValueError:
-            raise CartError(_('Please enter numbers only.'))
-        if amount < 0:
-            raise CartError(_('Please enter positive numbers only.'))
-        elif amount == 0:
-            return
-
-        if key.startswith('item_'):
-            try:
-                return {
-                    'item': int(parts[1]),
-                    'variation': None,
-                    'count': amount,
-                    'price': price,
-                    'voucher': voucher,
-                    'voucher_ignore_if_redeemed': voucher_ignore_if_redeemed,
-                    'subevent': subevent
-                }
-            except ValueError:
-                raise CartError(_('Please enter numbers only.'))
-        elif key.startswith('variation_'):
-            try:
-                return {
-                    'item': int(parts[1]),
-                    'variation': int(parts[2]),
-                    'count': amount,
-                    'price': price,
-                    'voucher': voucher,
-                    'voucher_ignore_if_redeemed': voucher_ignore_if_redeemed,
-                    'subevent': subevent
-                }
-            except ValueError:
-                raise CartError(_('Please enter numbers only.'))
-
-    def _items_from_post_data(self):
-        """
-        Parses the POST data and returns a list of dictionaries
-        """
-
-        # Compatibility patch that makes the frontend code a lot easier
-        req_items = list(self.request.POST.lists())
-        if '_voucher_item' in self.request.POST and '_voucher_code' in self.request.POST:
-            req_items.append((
-                '%s' % self.request.POST['_voucher_item'], ('1',)
-            ))
             pass
 
-        items = []
-        if 'raw' in self.request.POST:
-            items += json.loads(self.request.POST.get("raw"))
-        for key, values in req_items:
-            for value in values:
-                try:
-                    item = self._item_from_post_value(key, value, self.request.POST.get('_voucher_code'),
-                                                      voucher_ignore_if_redeemed=self.request.POST.get('_voucher_ignore_if_redeemed') == 'on')
-                except CartError as e:
-                    messages.error(self.request, str(e))
-                    return
-                if item:
-                    items.append(item)
+    if not key.startswith('item_') and not key.startswith('variation_') and not key.startswith('seat_'):
+        return
 
-        if len(items) == 0:
-            messages.warning(self.request, _('You did not select any products.'))
-            return []
-        return items
+    parts = key.split("_")
+    price = request.POST.get('price_' + "_".join(parts[1:]), "")
+
+    if key.startswith('seat_'):
+        try:
+            return {
+                'item': int(parts[1]),
+                'variation': int(parts[2]) if len(parts) > 2 else None,
+                'count': 1,
+                'seat': value,
+                'price': price,
+                'voucher': voucher,
+                'voucher_ignore_if_redeemed': voucher_ignore_if_redeemed,
+                'subevent': subevent
+            }
+        except ValueError:
+            raise CartError(_('Please enter numbers only.'))
+
+    try:
+        amount = int(value)
+    except ValueError:
+        raise CartError(_('Please enter numbers only.'))
+    if amount < 0:
+        raise CartError(_('Please enter positive numbers only.'))
+    elif amount == 0:
+        return
+
+    if key.startswith('item_'):
+        try:
+            return {
+                'item': int(parts[1]),
+                'variation': None,
+                'count': amount,
+                'price': price,
+                'voucher': voucher,
+                'voucher_ignore_if_redeemed': voucher_ignore_if_redeemed,
+                'subevent': subevent
+            }
+        except ValueError:
+            raise CartError(_('Please enter numbers only.'))
+    elif key.startswith('variation_'):
+        try:
+            return {
+                'item': int(parts[1]),
+                'variation': int(parts[2]),
+                'count': amount,
+                'price': price,
+                'voucher': voucher,
+                'voucher_ignore_if_redeemed': voucher_ignore_if_redeemed,
+                'subevent': subevent
+            }
+        except ValueError:
+            raise CartError(_('Please enter numbers only.'))
+
+
+def _items_from_post_data(request, warn_if_empty=True):
+    """
+    Parses the POST data and returns a list of dictionaries
+    """
+
+    # Compatibility patch that makes the frontend code a lot easier
+    req_items = list(request.POST.lists())
+    if '_voucher_item' in request.POST and '_voucher_code' in request.POST:
+        req_items.append((
+            '%s' % request.POST['_voucher_item'], ('1',)
+        ))
+        pass
+
+    items = []
+    if 'raw' in request.POST:
+        items += json.loads(request.POST.get("raw"))
+    for key, values in req_items:
+        for value in values:
+            try:
+                item = _item_from_post_value(request, key, value, request.POST.get('_voucher_code'),
+                                             voucher_ignore_if_redeemed=request.POST.get('_voucher_ignore_if_redeemed') == 'on')
+            except CartError as e:
+                messages.error(request, str(e))
+                return
+            if item:
+                items.append(item)
+
+    if len(items) == 0 and warn_if_empty:
+        messages.warning(request, _('You did not select any products.'))
+        return []
+    return items
 
 
 @scopes_disabled()
@@ -534,7 +544,7 @@ class CartAdd(EventViewMixin, CartActionMixin, AsyncAction, View):
             cs = cart_session(request)
             widget_data = cs.get('widget_data', {})
 
-        items = self._items_from_post_data()
+        items = _items_from_post_data(self.request)
         if items:
             return self.do(self.request.event.id, items, cart_id, translation.get_language(),
                            self.invoice_address.pk, widget_data, self.request.sales_channel.identifier,

src/pretix/presale/views/event.py

@@ -111,7 +111,8 @@ def item_group_by_category(items):
     )
 
 
-def get_grouped_items(event, *, channel: SalesChannel, subevent=None, voucher=None, require_seat=0, base_qs=None, allow_addons=False,
+def get_grouped_items(event, *, channel: SalesChannel, subevent=None, voucher=None, require_seat=0, base_qs=None,
+                      allow_addons=False, allow_cross_sell=False,
                       quota_cache=None, filter_items=None, filter_categories=None, memberships=None,
                       ignore_hide_sold_out_for_item_ids=None):
     base_qs_set = base_qs is not None
@@ -193,7 +194,9 @@ def get_grouped_items(event, *, channel: SalesChannel, subevent=None, voucher=No
         )
     )
 
-    items = base_qs.using(settings.DATABASE_REPLICA).filter_available(channel=channel.identifier, voucher=voucher, allow_addons=allow_addons).select_related(
+    items = base_qs.using(settings.DATABASE_REPLICA).filter_available(
+        channel=channel.identifier, voucher=voucher, allow_addons=allow_addons, allow_cross_sell=allow_cross_sell
+    ).select_related(
         'category', 'tax_rule',  # for re-grouping
         'hidden_if_available',
     ).prefetch_related(

src/pretix/static/pretixcontrol/js/ui/main.js

@@ -397,6 +397,9 @@ var form_handlers = function (el) {
                     enabled = !enabled;
                 }
                 var $toggling = dependent;
+                if (dependent.attr("data-disable-dependent")) {
+                    $toggling.attr('disabled', !enabled);
+                }
                 if (dependent.get(0).tagName.toLowerCase() !== "div") {
                     $toggling = dependent.closest('.form-group');
                 }
@@ -411,8 +414,9 @@ var form_handlers = function (el) {
                 }
             };
         update();
-        dependency.closest('.form-group').find('[name=' + dependency.attr("name") + ']').on("change", update);
-        dependency.closest('.form-group').find('[name=' + dependency.attr("name") + ']').on("dp.change", update);
+        dependency.each(function() {
+            $(this).closest('.form-group').find('[name=' + $(this).attr("name") + ']').on("change dp.change", update);
+        })
     });
 
     el.find("input[data-required-if], select[data-required-if], textarea[data-required-if]").each(function () {

src/pretix/static/pretixcontrol/scss/_forms.scss

@@ -567,7 +567,7 @@ table td > .checkbox input[type="checkbox"] {
     }
   }
 }
-.form-horizontal .big-radio {
+.form-horizontal .big-radio, .form-horizontal .big-radio-wrapper .radio {
   border: 1px solid #ccc;
   border-bottom: 0;
   padding: 0;
@@ -588,6 +588,17 @@ table td > .checkbox input[type="checkbox"] {
     border-bottom: 1px solid #ccc;
   }
 }
+.form-horizontal .control-label:has(+.big-radio-wrapper),
+.form-horizontal .control-label:has(+div > .big-radio) {
+  padding-top: 16px;
+}
+.form-horizontal .big-radio-wrapper .radio label {
+  font-weight: bold;
+}
+.form-horizontal .big-radio-wrapper .radio label > span {
+  display: block;
+  font-weight: normal;
+}
 .accordion-radio {
   display: block;
   margin: 0;

src/pretix/static/pretixpresale/scss/_checkout.scss

@@ -102,6 +102,17 @@
     }
   }
 }
+.cross-selling .panel-body h3 {
+  font-size: 21px;
+  line-height: inherit;
+  margin-top: 20px;
+}
+.cross-selling .panel-body > *:first-child > h3:first-child {
+  margin-top: 0;
+}
+.cross-selling .panel-body h3 small {
+  padding-left: 20px;
+}
 
 
 .panel-confirm {

src/tests/api/test_items.py

@@ -128,7 +128,10 @@ TEST_CATEGORY_RES = {
     "description": {"en": ""},
     "internal_name": None,
     "position": 0,
-    "is_addon": False
+    "is_addon": False,
+    "cross_selling_mode": None,
+    "cross_selling_condition": None,
+    "cross_selling_match_products": [],
 }
 
 
@@ -211,6 +214,44 @@ def test_category_update(token_client, organizer, event, team, category):
         assert ItemCategory.objects.get(pk=category.pk).name == {"en": "Test"}
 
 
+@pytest.mark.django_db
+def test_category_update_cross_selling_options(token_client, organizer, event, team, category):
+    resp = token_client.patch(
+        '/api/v1/organizers/{}/events/{}/categories/{}/'.format(organizer.slug, event.slug, category.pk),
+        {
+            "cross_selling_mode": "both",
+        },
+        format='json'
+    )
+    assert resp.status_code == 200
+    with scopes_disabled():
+        assert ItemCategory.objects.get(pk=category.pk).cross_selling_mode == 'both'
+
+    resp = token_client.patch(
+        '/api/v1/organizers/{}/events/{}/categories/{}/'.format(organizer.slug, event.slug, category.pk),
+        {
+            "cross_selling_mode": "something",
+        },
+        format='json'
+    )
+    assert resp.status_code == 400
+    with scopes_disabled():
+        assert ItemCategory.objects.get(pk=category.pk).cross_selling_mode == 'both'
+
+    resp = token_client.patch(
+        '/api/v1/organizers/{}/events/{}/categories/{}/'.format(organizer.slug, event.slug, category.pk),
+        {
+            "is_addon": True,
+        },
+        format='json'
+    )
+    assert resp.status_code == 400
+    assert 'mutually exclusive' in str(resp.data)
+    with scopes_disabled():
+        assert ItemCategory.objects.get(pk=category.pk).cross_selling_mode == 'both'
+        assert ItemCategory.objects.get(pk=category.pk).is_addon is False
+
+
 @pytest.mark.django_db
 def test_category_update_wrong_event(token_client, organizer, event2, category):
     resp = token_client.patch(

src/tests/base/test_cross_selling.py

@@ -0,0 +1,897 @@
+#
+# This file is part of pretix (Community Edition).
+#
+# Copyright (C) 2014-2020 Raphael Michel and contributors
+# Copyright (C) 2020-2021 rami.io GmbH and contributors
+#
+# This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General
+# Public License as published by the Free Software Foundation in version 3 of the License.
+#
+# ADDITIONAL TERMS APPLY: Pursuant to Section 7 of the GNU Affero General Public License, additional terms are
+# applicable granting you additional permissions and placing additional restrictions on your usage of this software.
+# Please refer to the pretix LICENSE file to obtain the full terms applicable to this work. If you did not receive
+# this file, see <https://pretix.eu/about/en/license>.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
+# <https://www.gnu.org/licenses/>.
+#
+import datetime
+import re
+from decimal import Decimal
+from typing import List, Tuple
+
+import pytest
+from django.utils.timezone import now
+from django_scopes import scopes_disabled
+from freezegun import freeze_time
+from tests import assert_num_queries
+
+from pretix.base.models import CartPosition, Discount, Event, Organizer
+from pretix.base.services.cross_selling import CrossSellingService
+
+
+@pytest.fixture
+def event():
+    o = Organizer.objects.create(name='Dummy', slug='dummy')
+    event = Event.objects.create(
+        organizer=o, name='Dummy', slug='dummy',
+        date_from=now()
+    )
+    return event
+
+
+@pytest.fixture
+@freeze_time("2020-01-01 10:00:00+01:00")
+def eventseries():
+    o = Organizer.objects.create(name='Dummy', slug='dummy')
+    start = now()
+    event = Event.objects.create(
+        organizer=o, name='Dummy', slug='dummy',
+        date_from=start, has_subevents=True
+    )
+    event.subevents.create(name='Date1', date_from=start + datetime.timedelta(hours=1), active=True)
+    event.subevents.create(name='Date2', date_from=start + datetime.timedelta(hours=2), active=True)
+    event.subevents.create(name='Date3', date_from=start + datetime.timedelta(hours=3), active=True)
+    return event
+
+
+def pattern(regex, **kwargs):
+    return re.compile(regex), kwargs
+
+
+cond_suffix = [
+    pattern(r" in the same subevent$", subevent_mode=Discount.SUBEVENT_MODE_SAME),
+    pattern(r" in distinct subevents$", subevent_mode=Discount.SUBEVENT_MODE_DISTINCT),
+]
+cond_patterns = [
+    pattern(r"^Buy at least (?P<condition_min_count>\d+) of (?P<condition_limit_products>.+)$",
+            condition_all_products=False),
+    pattern(r"^Buy at least (?P<condition_min_count>\d+) products$",
+            condition_all_products=True),
+    pattern(r"^Spend at least (?P<condition_min_value>\d+)\$$",
+            condition_all_products=True),
+    pattern(r"^For every (?P<condition_min_count>\d+) of (?P<condition_limit_products>.+)$",
+            condition_all_products=False),
+    pattern(r"^For every (?P<condition_min_count>\d+) products$",
+            condition_all_products=True),
+]
+benefit_patterns = [
+    pattern(r"^get (?P<benefit_discount_matching_percent>\d+)% discount on them\.$",
+            benefit_same_products=True),
+    pattern(r"^get (?P<benefit_discount_matching_percent>\d+)% discount on everything\.$",
+            benefit_same_products=True),
+    pattern(r"^get (?P<benefit_discount_matching_percent>\d+)% discount on "
+            r"(?P<benefit_only_apply_to_cheapest_n_matches>\d+) of them\.$",
+            benefit_same_products=True),
+    pattern(r"^get (?P<benefit_discount_matching_percent>\d+)% discount on "
+            r"(?P<benefit_only_apply_to_cheapest_n_matches>\d+) of (?P<benefit_limit_products>.+)\.$",
+            benefit_same_products=False),
+    pattern(r"^get (?P<benefit_discount_matching_percent>\d+)% discount on "
+            r"(?P<benefit_limit_products>.+)\.$",
+            benefit_same_products=False),
+]
+
+
+def make_discount(description, event: Event):
+    condition, benefit = description.split(', ')
+
+    d = Discount(event=event, internal_name=description)
+    d.save()
+
+    def apply(patterns: List[Tuple[re.Pattern, dict]], input):
+        for regex, options in patterns:
+            m = regex.search(input)
+            if m:
+                fields = m.groupdict()
+                for k, v in [*fields.items(), *options.items()]:
+                    if '_limit_products' in k:
+                        getattr(d, k).set([event.items.get(name=v)])
+                    else:
+                        setattr(d, k, v)
+                input = input[:m.start(0)] + input[m.endpos:]
+        if input != '':
+            raise Exception("Unable to parse '{}'".format(input))
+
+    apply(cond_suffix + cond_patterns, condition)
+    apply(benefit_patterns, benefit)
+
+    d.full_clean()
+    d.save()
+    return d
+
+
+def validate_discount_rule(
+        d,
+        subevent_mode=Discount.SUBEVENT_MODE_MIXED,
+        condition_all_products=True,
+        condition_limit_products=[],
+        condition_apply_to_addons=True,
+        condition_ignore_voucher_discounted=False,
+        condition_min_count=0,
+        condition_min_value=Decimal('0.00'),
+        benefit_same_products=True,
+        benefit_limit_products=[],
+        benefit_discount_matching_percent=Decimal('0.00'),
+        benefit_only_apply_to_cheapest_n_matches=None,
+        benefit_apply_to_addons=True,
+        benefit_ignore_voucher_discounted=False):
+    assert d.subevent_mode == subevent_mode
+    assert d.condition_all_products == condition_all_products
+    assert [str(p.name) for p in d.condition_limit_products.all()] == condition_limit_products
+    assert d.condition_apply_to_addons == condition_apply_to_addons
+    assert d.condition_ignore_voucher_discounted == condition_ignore_voucher_discounted
+    assert d.condition_min_count == condition_min_count
+    assert d.condition_min_value == condition_min_value
+    assert d.benefit_same_products == benefit_same_products
+    assert [str(p.name) for p in d.benefit_limit_products.all()] == benefit_limit_products
+    assert d.benefit_discount_matching_percent == benefit_discount_matching_percent
+    assert d.benefit_only_apply_to_cheapest_n_matches == benefit_only_apply_to_cheapest_n_matches
+    assert d.benefit_apply_to_addons == benefit_apply_to_addons
+    assert d.benefit_ignore_voucher_discounted == benefit_ignore_voucher_discounted
+    return d
+
+
+@scopes_disabled()
+@pytest.mark.django_db
+def test_rule_parser(event):
+    # mixed_min_count_matching_percent
+    validate_discount_rule(
+        make_discount("Buy at least 3 products, get 20% discount on everything.", event),
+        subevent_mode=Discount.SUBEVENT_MODE_MIXED,
+        condition_min_count=3,
+        benefit_discount_matching_percent=Decimal('20.00')
+    )
+
+    # mixed_min_count_one_free
+    validate_discount_rule(
+        make_discount("For every 3 products, get 100% discount on 1 of them.", event),
+        subevent_mode=Discount.SUBEVENT_MODE_MIXED,
+        condition_min_count=3,
+        benefit_discount_matching_percent=Decimal('100.00'),
+        benefit_only_apply_to_cheapest_n_matches=1,
+    )
+
+    # mixed_min_value_matching_percent
+    validate_discount_rule(
+        make_discount("Spend at least 500$, get 20% discount on everything.", event),
+        subevent_mode=Discount.SUBEVENT_MODE_MIXED,
+        condition_min_value=Decimal('500.00'),
+        benefit_discount_matching_percent=Decimal('20.00')
+    )
+
+    # same_min_count_matching_percent
+    validate_discount_rule(
+        make_discount("Buy at least 3 products in the same subevent, get 20% discount on everything.", event),
+        subevent_mode=Discount.SUBEVENT_MODE_SAME,
+        condition_min_count=3,
+        benefit_discount_matching_percent=Decimal('20.00')
+    )
+
+    # same_min_count_one_free
+    validate_discount_rule(
+        make_discount("For every 3 products in the same subevent, get 100% discount on 1 of them.", event),
+        subevent_mode=Discount.SUBEVENT_MODE_SAME,
+        condition_min_count=3,
+        benefit_discount_matching_percent=Decimal('100.00'),
+        benefit_only_apply_to_cheapest_n_matches=1,
+    )
+
+    # same_min_value_matching_percent
+    validate_discount_rule(
+        make_discount("Spend at least 500$ in the same subevent, get 20% discount on everything.", event),
+        subevent_mode=Discount.SUBEVENT_MODE_SAME,
+        condition_min_value=Decimal('500.00'),
+        benefit_discount_matching_percent=Decimal('20.00')
+    )
+
+    # distinct_min_count_matching_percent
+    validate_discount_rule(
+        make_discount("Buy at least 3 products in distinct subevents, get 20% discount on everything.", event),
+        subevent_mode=Discount.SUBEVENT_MODE_DISTINCT,
+        condition_min_count=3,
+        benefit_discount_matching_percent=Decimal('20.00')
+    )
+
+    # distinct_min_count_one_free
+    validate_discount_rule(
+        make_discount("For every 3 products in distinct subevents, get 100% discount on 1 of them.", event),
+        subevent_mode=Discount.SUBEVENT_MODE_DISTINCT,
+        condition_min_count=3,
+        benefit_discount_matching_percent=Decimal('100.00'),
+        benefit_only_apply_to_cheapest_n_matches=1,
+    )
+
+    # distinct_min_count_two_free
+    validate_discount_rule(
+        make_discount("For every 3 products in distinct subevents, get 100% discount on 2 of them.", event),
+        subevent_mode=Discount.SUBEVENT_MODE_DISTINCT,
+        condition_min_count=3,
+        benefit_discount_matching_percent=Decimal('100.00'),
+        benefit_only_apply_to_cheapest_n_matches=2,
+    )
+
+
+def setup_items(event, category_name, category_type, cross_selling_condition, *items):
+    cat = event.categories.create(name=category_name)
+    cat.category_type = category_type
+    cat.cross_selling_condition = cross_selling_condition
+    cat.save()
+    for name, price in items:
+        item = cat.items.create(event=event, name=name, default_price=price)
+        for subevent in event.subevents.all() if event.has_subevents else [None]:
+            quota = event.quotas.create(subevent=subevent)
+            quota.items.add(item)
+            quota.save()
+
+
+def split_table(txt):
+    return [
+        re.split(r"\s{3,}", line.strip())
+        for line in txt.split("\n")[1:]
+        if line.strip() != ""
+    ]
+
+
+def check_cart_behaviour(event, cart_contents, recommendations, expect_num_queries=None):
+    cart_contents = split_table(cart_contents)
+    subevent_map = {str(se.name): se.pk for se in event.subevents.all()}
+    positions = [
+        CartPosition(
+            item_id=event.items.get(name=item_name).pk,
+            subevent_id=subevent_map.get(subevent_name),
+            line_price_gross=Decimal(regular_price), addon_to=None, is_bundled=False,
+            listed_price=Decimal(regular_price), price_after_voucher=Decimal(regular_price)
+        ) for (item_name, regular_price, expected_discounted_price, subevent_name) in cart_contents
+    ]
+    expected_recommendations = split_table(recommendations)
+
+    event.organizer.cache.clear()
+    event.cache.clear()
+    event.refresh_from_db()
+    service = CrossSellingService(event, event.organizer.sales_channels.get(identifier='web'), positions, None)
+    if expect_num_queries:
+        with assert_num_queries(expect_num_queries):
+            result = service.get_data()
+    else:
+        result = service.get_data()
+    result_recommendations = [
+        [str(category.name) + (f' ({category.subevent_name})' if hasattr(category, 'subevent_name') else ''),
+         str(item.name),
+         str(item.original_price.gross.quantize(Decimal('0.00'))) if item.original_price else '-',
+         str(item.display_price.gross.quantize(Decimal('0.00'))),
+         str(item.order_max),
+         form_prefix or '-']
+        for category, items, form_prefix in result
+        for item in items
+    ]
+
+    assert result_recommendations == expected_recommendations
+    assert [str(price) for price, discount in service._discounted_prices] == [
+        expected_discounted_price for (item_name, regular_price, expected_discounted_price, form_prefix) in cart_contents]
+
+
+@scopes_disabled()
+@pytest.mark.django_db
+def test_2f1r_discount_cross_selling(event):
+    setup_items(event, 'Tickets', 'both', 'discounts',
+                ('Regular Ticket', '42.00'),
+                ('Reduced Ticket', '23.00'),
+                )
+    make_discount('For every 2 of Regular Ticket, get 50% discount on 1 of Reduced Ticket.', event)
+
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        Tickets     Reduced Ticket      23.00                11.50            1       -
+        '''
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+
+        Reduced Ticket    23.00          11.50       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        '''
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+
+        Reduced Ticket    23.00          11.50       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        '''
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        Tickets     Reduced Ticket      23.00                11.50            2       -
+        '''
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+
+        Reduced Ticket    23.00          11.50       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        Tickets     Reduced Ticket      23.00                11.50            1       -
+        '''
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+
+        Reduced Ticket    23.00          11.50       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        Tickets     Reduced Ticket      23.00                11.50            1       -
+        '''
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+
+        Reduced Ticket    23.00          11.50       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        Tickets     Reduced Ticket      23.00                11.50            2       -
+        '''
+    )
+
+
+@scopes_disabled()
+@pytest.mark.django_db
+@freeze_time("2020-01-01 10:00:00+01:00")
+def test_2f1r_discount_cross_selling_eventseries_mixed(eventseries):
+    setup_items(eventseries, 'Tickets', 'both', 'discounts',
+                ('Regular Ticket', '42.00'),
+                ('Reduced Ticket', '23.00'),
+                )
+    make_discount('For every 2 of Regular Ticket, get 50% discount on 1 of Reduced Ticket.', eventseries)
+    prefix_date1 = f"subevent_{eventseries.subevents.get(name='Date1').pk}_"
+    prefix_date2 = f"subevent_{eventseries.subevents.get(name='Date2').pk}_"
+
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        ''',
+        recommendations=f'''                                                 Price     Discounted Price    Max Count   Prefix
+        Tickets (Date1 - Wed, Jan. 1st, 2020 10:00)     Reduced Ticket      23.00                11.50            1   {prefix_date1}
+        '''
+    )
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date2
+        ''',
+        recommendations=f'''                                                 Price     Discounted Price    Max Count   Prefix
+        Tickets (Date1 - Wed, Jan. 1st, 2020 10:00)     Reduced Ticket      23.00                11.50            1   {prefix_date1}
+        Tickets (Date2 - Wed, Jan. 1st, 2020 11:00)     Reduced Ticket      23.00                11.50            1   {prefix_date2}
+        '''
+    )
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date2
+
+        Reduced Ticket    23.00          11.50   Date1
+        ''',
+        recommendations='''                                                 Price     Discounted Price    Max Count   Prefix
+        '''
+    )
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date2
+        Regular Ticket    42.00          42.00   Date1
+
+        Reduced Ticket    23.00          11.50   Date1
+        ''',
+        recommendations='''                                                 Price     Discounted Price    Max Count   Prefix
+        '''
+    )
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date2
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date2
+        ''',
+        recommendations=f'''                                                 Price     Discounted Price    Max Count   Prefix
+        Tickets (Date1 - Wed, Jan. 1st, 2020 10:00)     Reduced Ticket      23.00                11.50            2   {prefix_date1}
+        Tickets (Date2 - Wed, Jan. 1st, 2020 11:00)     Reduced Ticket      23.00                11.50            2   {prefix_date2}
+        '''
+    )
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+
+        Reduced Ticket    23.00          11.50   Date1
+        ''',
+        recommendations=f'''                                                 Price     Discounted Price    Max Count   Prefix
+        Tickets (Date1 - Wed, Jan. 1st, 2020 10:00)     Reduced Ticket      23.00                11.50            1   {prefix_date1}
+        '''
+    )
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+
+        Reduced Ticket    23.00          11.50   Date1
+        ''',
+        recommendations=f'''                                                 Price     Discounted Price    Max Count   Prefix
+        Tickets (Date1 - Wed, Jan. 1st, 2020 10:00)     Reduced Ticket      23.00                11.50            1   {prefix_date1}
+        '''
+    )
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+
+        Reduced Ticket    23.00          11.50   Date1
+        ''',
+        recommendations=f'''                                                 Price     Discounted Price    Max Count   Prefix
+        Tickets (Date1 - Wed, Jan. 1st, 2020 10:00)     Reduced Ticket      23.00                11.50            2   {prefix_date1}
+        '''
+    )
+
+
+@scopes_disabled()
+@pytest.mark.django_db
+def test_2f1r_discount_cross_selling_eventseries_same(eventseries):
+    setup_items(eventseries, 'Tickets', 'both', 'discounts',
+                ('Regular Ticket', '42.00'),
+                ('Reduced Ticket', '23.00'),
+                )
+    make_discount('For every 2 of Regular Ticket in the same subevent, get 50% discount on 1 of Reduced Ticket.', eventseries)
+    prefix_date1 = f"subevent_{eventseries.subevents.get(name='Date1').pk}_"
+    prefix_date2 = f"subevent_{eventseries.subevents.get(name='Date2').pk}_"
+
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date2
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date2
+        ''',
+        recommendations=f'''                                                 Price     Discounted Price    Max Count   Prefix
+        Tickets (Date1 - Wed, Jan. 1st, 2020 10:00)     Reduced Ticket      23.00                11.50            1   {prefix_date1}
+        Tickets (Date2 - Wed, Jan. 1st, 2020 11:00)     Reduced Ticket      23.00                11.50            1   {prefix_date2}
+        '''
+    )
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date2
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date2
+        Regular Ticket    42.00          42.00   Date2
+        Regular Ticket    42.00          42.00   Date2
+        ''',
+        recommendations=f'''                                                 Price     Discounted Price    Max Count   Prefix
+        Tickets (Date1 - Wed, Jan. 1st, 2020 10:00)     Reduced Ticket      23.00                11.50            1   {prefix_date1}
+        Tickets (Date2 - Wed, Jan. 1st, 2020 11:00)     Reduced Ticket      23.00                11.50            2   {prefix_date2}
+        '''
+    )
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+
+        Reduced Ticket    23.00          11.50   Date1
+        ''',
+        recommendations=f'''                                                 Price     Discounted Price    Max Count   Prefix
+        Tickets (Date1 - Wed, Jan. 1st, 2020 10:00)     Reduced Ticket      23.00                11.50            1   {prefix_date1}
+        '''
+    )
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date2
+
+        Reduced Ticket    23.00          11.50   Date1
+        ''',
+        recommendations='''                                                 Price     Discounted Price    Max Count   Prefix
+        '''
+    )
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date2
+        Regular Ticket    42.00          42.00   Date2
+
+        Reduced Ticket    23.00          11.50   Date1
+        ''',
+        recommendations=f'''                                                 Price     Discounted Price    Max Count   Prefix
+        Tickets (Date2 - Wed, Jan. 1st, 2020 11:00)     Reduced Ticket      23.00                11.50            1   {prefix_date2}
+        '''
+    )
+
+
+@scopes_disabled()
+@pytest.mark.django_db
+def test_50percentoff_discount_cross_selling_eventseries_distinct(eventseries):
+    setup_items(eventseries, 'Tickets', 'both', 'discounts',
+                ('Regular Ticket', '42.00'),
+                ('Reduced Ticket', '23.00'),
+                )
+    make_discount('For every 2 of Regular Ticket in distinct subevents, get 50% discount on them.', eventseries)
+
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        Regular Ticket    42.00          42.00   Date1
+        ''',
+        recommendations='''                                                 Price     Discounted Price    Max Count   Prefix
+        '''
+    )
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          21.00   Date1
+        Regular Ticket    42.00          21.00   Date2
+        Regular Ticket    42.00          21.00   Date3
+        ''',
+        recommendations='''                                                 Price     Discounted Price    Max Count   Prefix
+        '''
+    )
+    check_cart_behaviour(
+        eventseries,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          21.00   Date1
+        Regular Ticket    42.00          21.00   Date2
+        Regular Ticket    42.00          21.00   Date3
+        Reduced Ticket    23.00          23.00   Date1
+        ''',
+        recommendations='''                                                 Price     Discounted Price    Max Count   Prefix
+        '''
+    )
+
+
+@scopes_disabled()
+@pytest.mark.django_db
+@pytest.mark.skip("currently unsupported (cannot give discount to specific product on minimum cart value)")
+def test_free_drinks(event):
+    setup_items(event, 'Tickets', 'normal', None,
+                ('Regular Ticket', '42.00'),
+                ('Reduced Ticket', '23.00'),
+                )
+    setup_items(event, 'Free Drinks', 'only', 'discounts',
+                ('Free Drinks', '50.00'),
+                )
+    make_discount('Spend at least 100$, get 100% discount on 1 of Free Drinks.', event)
+
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        '''
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        Free Drinks     Free Drinks     50.00                 0.00            1        -
+        '''
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Free Drinks       50.00           0.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        '''
+    )
+
+
+@scopes_disabled()
+@pytest.mark.django_db
+def test_five_tickets_one_free(event):
+    setup_items(event, 'Tickets', 'both', 'discounts',
+                ('Regular Ticket', '42.00'),
+                )
+    make_discount('For every 5 of Regular Ticket, get 100% discount on 1 of them.', event)
+    # we don't expect a recommendation here, as in the current implementation we only recommend based on discounts
+    # where the condition is already completely satisfied but no (or not enough) benefitting products are in the
+    # cart yet
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        '''
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00           0.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        '''
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00          42.00       0
+        Regular Ticket    42.00           0.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        '''
+    )
+
+
+@scopes_disabled()
+@pytest.mark.django_db
+@pytest.mark.parametrize("itemcount", [3, 10, 50])
+def test_query_count_many_items(event, itemcount):
+    setup_items(event, 'Tickets', 'both', 'discounts',
+                *[(f'Ticket {n}', '42.00') for n in range(itemcount)]
+                )
+    make_discount('For every 5 of Ticket 1, get 100% discount on 1 of Ticket 2.', event)
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Ticket 1          42.00          42.00       0
+        Ticket 1          42.00          42.00       0
+        Ticket 1          42.00          42.00       0
+        Ticket 1          42.00          42.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        ''',
+        expect_num_queries=8,
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Ticket 1          42.00          42.00       0
+        Ticket 1          42.00          42.00       0
+        Ticket 1          42.00          42.00       0
+        Ticket 1          42.00          42.00       0
+        Ticket 1          42.00          42.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        Tickets        Ticket 2         42.00                 0.00            1        -
+        ''',
+        expect_num_queries=9,
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Ticket 1          42.00          42.00       0
+        Ticket 1          42.00          42.00       0
+        Ticket 1          42.00          42.00       0
+        Ticket 1          42.00          42.00       0
+        Ticket 1          42.00          42.00       0
+        Ticket 1          42.00          42.00       0
+        Ticket 1          42.00          42.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        Tickets        Ticket 2         42.00                 0.00            1        -
+        ''',
+        expect_num_queries=9,
+    )
+
+
+@scopes_disabled()
+@pytest.mark.django_db
+@pytest.mark.parametrize("catcount", [1, 10, 50])
+def test_query_count_many_categories_and_discounts(event, catcount):
+    for n in range(1, catcount + 1):
+        setup_items(event, f'Category {n}', 'both', 'discounts',
+                    (f'Ticket {n}-A', '42.00'),
+                    (f'Ticket {n}-B', '42.00'),
+                    )
+        make_discount(f'For every 5 of Ticket {n}-A, get 100% discount on 1 of Ticket {n}-B.', event)
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        ''',
+        expect_num_queries=8,
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        Category 1     Ticket 1-B       42.00                 0.00            1        -
+        ''',
+        expect_num_queries=9,
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        Category 1     Ticket 1-B       42.00                 0.00            1        -
+        ''',
+        expect_num_queries=9,
+    )
+
+
+@scopes_disabled()
+@pytest.mark.django_db
+@pytest.mark.parametrize("catcount", [2, 10, 50])
+def test_query_count_many_cartpos(event, catcount):
+    for n in range(1, catcount + 1):
+        setup_items(event, f'Category {n}', 'both', 'discounts',
+                    (f'Ticket {n}-A', '42.00'),
+                    (f'Ticket {n}-B', '42.00'),
+                    )
+        make_discount(f'For every 5 of Ticket {n}-A, get 100% discount on 1 of Ticket {n}-B.', event)
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        ''',
+        expect_num_queries=8,
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        Category 1     Ticket 1-B       42.00                 0.00            1        -
+        ''',
+        expect_num_queries=9,
+    )
+    check_cart_behaviour(
+        event,
+        cart_contents=''' Price     Discounted   Subev
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 1-A        42.00          42.00       0
+        Ticket 2-A        42.00          42.00       0
+        Ticket 2-A        42.00          42.00       0
+        Ticket 2-A        42.00          42.00       0
+        Ticket 2-A        42.00          42.00       0
+        Ticket 2-A        42.00          42.00       0
+        ''',
+        recommendations='''             Price     Discounted Price    Max Count   Prefix
+        Category 1     Ticket 1-B       42.00                 0.00            1        -
+        Category 2     Ticket 2-B       42.00                 0.00            1        -
+        ''',
+        expect_num_queries=13,
+    )

src/tests/base/test_event_clone.py

@@ -65,6 +65,8 @@ def test_full_clone_same_organizer():
     item_meta = event.item_meta_properties.create(name="Bla")
     tax_rule = event.tax_rules.create(name="VAT", rate=19)
     category = event.categories.create(name="Tickets")
+    cross_sell_category = event.categories.create(name="Recommendations", cross_selling_mode="only",
+                                                  cross_selling_condition="products")
 
     q1 = event.quotas.create(name="Quota 1", size=5)
     q2 = event.quotas.create(name="Quota 2", size=0, closed=True)
@@ -88,6 +90,7 @@ def test_full_clone_same_organizer():
     q1.items.add(item1)
     q2.items.add(item2)
     q2.variations.add(item2v)
+    cross_sell_category.cross_selling_match_products.add(item1)
 
     event.discounts.create(internal_name="Fake discount")
     question1 = event.questions.create(question="Yes or no", type=Question.TYPE_BOOLEAN)
@@ -156,7 +159,7 @@ def test_full_clone_same_organizer():
     copied_item1 = copied_event.items.get(name=item1.name)
     copied_item2 = copied_event.items.get(name=item2.name)
     assert copied_item1.tax_rule == copied_event.tax_rules.get()
-    assert copied_item1.category == copied_event.categories.get()
+    assert copied_item1.category == copied_event.categories.get(name='Tickets')
     assert copied_item1.limit_sales_channels.get() == sc
     assert copied_item1.meta_data == item1.meta_data
     assert copied_item2.variations.get().meta_data == item2v.meta_data
@@ -166,7 +169,7 @@ def test_full_clone_same_organizer():
     assert copied_item2.variations.get().limit_sales_channels.get() == sc
     assert copied_item2.require_membership_types.count() == 1
     assert copied_item2.require_membership_types.get() == membership_type
-    assert copied_item1.addons.get().addon_category == copied_event.categories.get()
+    assert copied_item1.addons.get().addon_category == copied_event.categories.get(name='Tickets')
     assert copied_item1.bundles.get().bundled_item == copied_item2
     assert copied_item1.bundles.get().bundled_variation == copied_item2.variations.get()
     assert copied_item2.hidden_if_item_available == copied_item1
@@ -174,6 +177,9 @@ def test_full_clone_same_organizer():
     assert copied_q2.items.get() == copied_item2
     assert copied_q2.variations.get() == copied_item2.variations.get()
 
+    copied_cross_sell_category = copied_event.categories.get(name=cross_sell_category.name)
+    assert copied_cross_sell_category.cross_selling_match_products.get() == copied_item1
+
     copied_question1 = copied_event.questions.get(type=question1.type)
     copied_question2 = copied_event.questions.get(type=question2.type)
     assert copied_question2.dependency_question == copied_question1