Fix incorrect quota error when changing subevent and item of a position

src/pretix/base/services/orders.py

@@ -1224,6 +1224,27 @@ class OrderChangeManager:
         self._quotadiff.subtract(position.quotas)
         self._operations.append(self.SubeventOperation(position, subevent))
 
+    def change_item_and_subevent(self, position: OrderPosition, item: Item, variation: Optional[ItemVariation],
+                                 subevent: SubEvent):
+        if (not variation and item.has_variations) or (variation and variation.item_id != item.pk):
+            raise OrderError(self.error_messages['product_without_variation'])
+
+        price = get_price(item, variation, voucher=position.voucher, subevent=subevent,
+                          invoice_address=self._invoice_address)
+
+        if price is None:  # NOQA
+            raise OrderError(self.error_messages['product_invalid'])
+
+        new_quotas = (variation.quotas.filter(subevent=subevent)
+                      if variation else item.quotas.filter(subevent=subevent))
+        if not new_quotas:
+            raise OrderError(self.error_messages['quota_missing'])
+
+        self._quotadiff.update(new_quotas)
+        self._quotadiff.subtract(position.quotas)
+        self._operations.append(self.ItemOperation(position, item, variation))
+        self._operations.append(self.SubeventOperation(position, subevent))
+
     def regenerate_secret(self, position: OrderPosition):
         self._operations.append(self.RegenerateSecretOperation(position))
 

src/pretix/control/views/orders.py

@@ -1487,6 +1487,7 @@ class OrderChange(OrderView):
                     ocm.cancel(p)
                     continue
 
+                change_item = None
                 if p.form.cleaned_data['itemvar']:
                     if '-' in p.form.cleaned_data['itemvar']:
                         itemid, varid = p.form.cleaned_data['itemvar'].split('-')
@@ -1499,10 +1500,18 @@ class OrderChange(OrderView):
                     else:
                         variation = None
                     if item != p.item or variation != p.variation:
-                        ocm.change_item(p, item, variation)
+                        change_item = (item, variation)
 
+                change_subevent = None
                 if self.request.event.has_subevents and p.form.cleaned_data['subevent'] and p.form.cleaned_data['subevent'] != p.subevent:
-                    ocm.change_subevent(p, p.form.cleaned_data['subevent'])
+                    change_subevent = (p.form.cleaned_data['subevent'],)
+
+                if change_item is not None and change_subevent is not None:
+                    ocm.change_item_and_subevent(p, *change_item, *change_subevent)
+                elif change_item is not None:
+                    ocm.change_item(p, *change_item)
+                elif change_subevent is not None:
+                    ocm.change_subevent(p, *change_subevent)
 
                 if p.seat and p.form.cleaned_data['seat'] and p.form.cleaned_data['seat'] != p.seat.seat_guid:
                     ocm.change_seat(p, p.form.cleaned_data['seat'])