Avoid showing empty add-on step (Z#23167007)

Do not show add-on step if none of the add-on items is available. This
is not a perfect solution, there are still cases where an empty add-on
step will show up, e.g.:

- Products disabled only for a specific subevent
- Sold-out add-ons combined with the "hide sold-out products" flag

src/pretix/base/models/event.py

@@ -313,38 +313,9 @@ class EventMixin:
             items=GroupConcat('pk', delimiter=',')
         ).values('items')
 
-        q_variation = (
-            Q(active=True)
-            & Q(Q(available_from__isnull=True) | Q(available_from__lte=time_machine_now()))
-            & Q(Q(available_until__isnull=True) | Q(available_until__gte=time_machine_now()))
-            & Q(item__active=True)
-            & Q(Q(item__available_from__isnull=True) | Q(item__available_from__lte=time_machine_now()))
-            & Q(Q(item__available_until__isnull=True) | Q(item__available_until__gte=time_machine_now()))
-            & Q(Q(item__category__isnull=True) | Q(item__category__is_addon=False))
-            & Q(item__require_bundling=False)
-            & Q(quotas__pk=OuterRef('pk'))
-        )
-
-        if isinstance(channel, str):
-            q_variation &= Q(Q(all_sales_channels=True) | Q(limit_sales_channels__identifier=channel))
-            q_variation &= Q(Q(item__all_sales_channels=True) | Q(item__limit_sales_channels__identifier=channel))
-        else:
-            q_variation &= Q(Q(all_sales_channels=True) | Q(limit_sales_channels=channel))
-            q_variation &= Q(Q(item__all_sales_channels=True) | Q(item__limit_sales_channels=channel))
-
-        if voucher:
-            if voucher.variation_id:
-                q_variation &= Q(pk=voucher.variation_id)
-            elif voucher.item_id:
-                q_variation &= Q(item_id=voucher.item_id)
-            elif voucher.quota_id:
-                q_variation &= Q(quotas__in=[voucher.quota_id])
-
-        if not voucher or not voucher.show_hidden_items:
-            q_variation &= Q(hide_without_voucher=False)
-            q_variation &= Q(item__hide_without_voucher=False)
-
-        sq_active_variation = ItemVariation.objects.filter(q_variation).order_by().values_list('quotas__pk').annotate(
+        sq_active_variation = ItemVariation.objects.filter_available(channel=channel, voucher=voucher).filter(
+            Q(quotas__pk=OuterRef('pk'))
+        ).order_by().values_list('quotas__pk').annotate(
             items=GroupConcat('pk', delimiter=',')
         ).values('items')
         quota_base_qs = Quota.objects.using(settings.DATABASE_REPLICA).filter(

src/pretix/base/models/items.py

@@ -303,6 +303,48 @@ def filter_available(qs, channel='web', voucher=None, allow_addons=False):
     return qs.filter(q)
 
 
+def filter_variations_available(qs, channel='web', voucher=None, allow_addons=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
+
+    assert isinstance(channel, (SalesChannel, str))
+    q = (
+        Q(active=True)
+        & Q(Q(available_from__isnull=True) | Q(available_from__lte=time_machine_now()))
+        & Q(Q(available_until__isnull=True) | Q(available_until__gte=time_machine_now()))
+        & Q(item__active=True)
+        & Q(Q(item__available_from__isnull=True) | Q(item__available_from__lte=time_machine_now()))
+        & Q(Q(item__available_until__isnull=True) | Q(item__available_until__gte=time_machine_now()))
+        & Q(Q(item__category__isnull=True) | Q(item__category__is_addon=False))
+        & Q(item__require_bundling=False)
+    )
+
+    if isinstance(channel, str):
+        q &= Q(Q(all_sales_channels=True) | Q(limit_sales_channels__identifier=channel))
+        q &= Q(Q(item__all_sales_channels=True) | Q(item__limit_sales_channels__identifier=channel))
+    else:
+        q &= Q(Q(all_sales_channels=True) | Q(limit_sales_channels=channel))
+        q &= Q(Q(item__all_sales_channels=True) | Q(item__limit_sales_channels=channel))
+
+    if not allow_addons:
+        q &= Q(Q(item__category__isnull=True) | Q(item__category__is_addon=False))
+
+    if voucher:
+        if voucher.variation_id:
+            q &= Q(pk=voucher.variation_id)
+        elif voucher.item_id:
+            q &= Q(item_id=voucher.item_id)
+        elif voucher.quota_id:
+            q &= Q(quotas__in=[voucher.quota_id])
+
+    if not voucher or not voucher.show_hidden_items:
+        q &= Q(hide_without_voucher=False)
+        q &= Q(item__hide_without_voucher=False)
+
+    return qs.filter(q)
+
+
 class ItemQuerySet(models.QuerySet):
     def filter_available(self, channel='web', voucher=None, allow_addons=False):
         return filter_available(self, channel, voucher, allow_addons)
@@ -317,6 +359,20 @@ class ItemQuerySetManager(ScopedManager(organizer='event__organizer').__class__)
         return filter_available(self.get_queryset(), channel, voucher, allow_addons)
 
 
+class ItemVariationQuerySet(models.QuerySet):
+    def filter_available(self, channel='web', voucher=None, allow_addons=False):
+        return filter_variations_available(self, channel, voucher, allow_addons)
+
+
+class ItemVariationQuerySetManager(ScopedManager(organizer='item__event__organizer').__class__):
+    def __init__(self):
+        super().__init__()
+        self._queryset_class = ItemVariationQuerySet
+
+    def filter_available(self, channel='web', voucher=None, allow_addons=False):
+        return filter_variations_available(self.get_queryset(), channel, voucher, allow_addons)
+
+
 class Item(LoggedModel):
     """
     An item is a thing which can be sold. It belongs to an event and may or may not belong to a category.
@@ -1199,7 +1255,7 @@ class ItemVariation(models.Model):
         help_text=_('This text will be shown by the check-in app if a ticket of this type is scanned.')
     )
 
-    objects = ScopedManager(organizer='item__event__organizer')
+    objects = ItemVariationQuerySetManager()
 
     class Meta:
         verbose_name = _("Product variation")

src/pretix/presale/checkoutflow.py

@@ -56,7 +56,7 @@ from django.views.generic.base import TemplateResponseMixin
 from django_scopes import scopes_disabled
 
 from pretix.base.models import Customer, Membership, Order
-from pretix.base.models.items import Question
+from pretix.base.models.items import ItemAddOn, ItemVariation, Question
 from pretix.base.models.orders import (
     InvoiceAddress, OrderPayment, QuestionAnswer,
 )
@@ -486,9 +486,33 @@ class AddOnsStep(CartMixin, AsyncAction, TemplateFlowStep):
     label = pgettext_lazy('checkoutflow', 'Add-on products')
     icon = 'puzzle-piece'
 
+    def _is_applicable(self, request):
+        categories = set(ItemAddOn.objects.filter(
+            base_item_id__in=get_cart(request).values_list("item_id", flat=True)
+        ).values_list("addon_category_id", flat=True))
+        if not categories:
+            return False
+
+        has_available_addons = (
+            self.event.items.filter_available(
+                channel=request.sales_channel,
+                allow_addons=True
+            ).filter(
+                variations__isnull=True,
+                category__in=categories,
+            ).exists() or ItemVariation.objects.filter_available(
+                channel=request.sales_channel,
+                allow_addons=True
+            ).filter(
+                item__event=self.event,
+                item__category__in=categories,
+            )
+        )
+        return has_available_addons
+
     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()
+            request._checkoutflow_addons_applicable = self._is_applicable(request)
         return request._checkoutflow_addons_applicable
 
     def is_completed(self, request, warn=False):