Cross selling (#4185)

Product categories can now be marked as "cross-selling categories", causing them to 
appear in the add-on checkout step as additional recommendations, depending on 
their cross-selling visibility (always, only if certain products are already in the cart, or 
only if they qualify for a discount according to discount rules).

---------

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

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):