Optionally allow self-service order changes after check-in

src/pretix/base/models/orders.py

@@ -626,7 +626,10 @@ class Order(LockModel, LoggedModel):
                 has_checkin=Exists(Checkin.objects.filter(position_id=OuterRef('pk')))
             ).select_related('item').prefetch_related('issued_gift_cards')
         )
-        cancelable = all([op.item.allow_cancel and not op.has_checkin for op in positions])
+        if self.event.settings.change_allow_user_if_checked_in:
+            cancelable = all([op.item.allow_cancel for op in positions])
+        else:
+            cancelable = all([op.item.allow_cancel and not op.has_checkin for op in positions])
         if not cancelable or not positions:
             return False
         for op in positions:

src/pretix/base/services/orders.py

@@ -189,6 +189,7 @@ error_messages = {
         'min'
     ),
     'addon_no_multi': gettext_lazy('You can select every add-ons from the category %(cat)s for the product %(base)s at most once.'),
+    'addon_already_checked_in': gettext_lazy('You cannot remove the position %(addon)s since it has already been checked in.'),
 }
 
 logger = logging.getLogger(__name__)
@@ -1896,6 +1897,12 @@ class OrderChangeManager:
                     for a in current_addons[cp][k][:current_num - input_num]:
                         if a.canceled:
                             continue
+                        if a.checkins.exists():
+                            raise OrderError(
+                                error_messages['addon_already_checked_in'] % {
+                                    'addon': str(a.item.name),
+                                }
+                            )
                         self.cancel(a)
 
     def _check_seats(self):

src/pretix/base/settings.py

@@ -1484,6 +1484,19 @@ DEFAULTS = {
             label=_("Do not allow changes after"),
         )
     },
+    'change_allow_user_if_checked_in': {
+        'default': 'False',
+        'type': bool,
+        'form_class': forms.BooleanField,
+        'serializer_class': serializers.BooleanField,
+        'form_kwargs': dict(
+            label=_("Allow change even though the ticket has already been checked in"),
+            help_text=_("By default, order changes are disabled after any ticket in the order has been checked in. "
+                        "If you check this box, this requirement is lifted. It is still not possible to remove an "
+                        "add-on product that has already been checked in individually. Use with care, and preferably "
+                        "only in combination with a limitation on price changes above."),
+        )
+    },
     'change_allow_attendee': {
         'default': 'False',
         'type': bool,