Allow attendees to change selected add-ons of same price (#3150)

src/pretix/base/models/orders.py

@@ -2555,6 +2555,27 @@ class OrderPosition(AbstractPosition):
                 attach_tickets=True
             )
 
+    @property
+    @scopes_disabled()
+    def attendee_change_allowed(self) -> bool:
+        """
+        Returns whether or not this order can be changed by the attendee.
+        """
+        from .items import ItemAddOn
+
+        if not self.event.settings.change_allow_attendee or not self.order.user_change_allowed:
+            return False
+
+        positions = list(
+            self.order.positions.filter(Q(pk=self.pk) | Q(addon_to_id=self.pk)).annotate(
+                has_variations=Exists(ItemVariation.objects.filter(item_id=OuterRef('item_id'))),
+            ).select_related('item').prefetch_related('issued_gift_cards')
+        )
+        return (
+            (self.order.event.settings.change_allow_user_variation and any([op.has_variations for op in positions])) or
+            (self.order.event.settings.change_allow_user_addons and ItemAddOn.objects.filter(base_item_id__in=[op.item_id for op in positions]).exists())
+        )
+
 
 class Transaction(models.Model):
     """

src/pretix/base/settings.py

@@ -1484,6 +1484,19 @@ DEFAULTS = {
             label=_("Do not allow changes after"),
         )
     },
+    'change_allow_attendee': {
+        'default': 'False',
+        'type': bool,
+        'form_class': forms.BooleanField,
+        'serializer_class': serializers.BooleanField,
+        'form_kwargs': dict(
+            label=_("Allow individual attendees to change their ticket"),
+            help_text=_("By default, only the person who ordered the tickets can make any changes. If you check this "
+                        "box, individual attendees can also make changes. However, individual attendees can always "
+                        "only make changes that do not change the total price of the order. Such changes can always "
+                        "only be made by the main customer."),
+        )
+    },
     'cancel_allow_user': {
         'default': 'True',
         'type': bool,