API: Fix crash with deletion of cart positions with add-ons

src/pretix/api/views/cart.py

@@ -92,6 +92,11 @@ class CartPositionViewSet(CreateModelMixin, DestroyModelMixin, viewsets.ReadOnly
     def perform_create(self, serializer):
         raise NotImplementedError()
 
+    @transaction.atomic()
+    def perform_destroy(self, instance):
+        instance.addons.all().delete()
+        instance.delete()
+
     def _require_locking(self, quota_diff, voucher_use_diff, seat_diff):
         if voucher_use_diff or seat_diff:
             # If any vouchers or seats are used, we lock to make sure we don't redeem them to often

src/tests/api/test_cart.py

@@ -167,6 +167,12 @@ def test_cp_delete(token_client, organizer, event, item, taxrule, question):
             datetime=datetime.datetime(2018, 6, 11, 10, 0, 0, 0, tzinfo=UTC),
             expires=datetime.datetime(2018, 6, 11, 10, 0, 0, 0, tzinfo=UTC)
         )
+        CartPosition.objects.create(
+            event=event, cart_id="aaa@api", item=item, addon_to=cr,
+            price=23, attendee_name_parts={'full_name': 'Peter'},
+            datetime=datetime.datetime(2018, 6, 11, 10, 0, 0, 0, tzinfo=UTC),
+            expires=datetime.datetime(2018, 6, 11, 10, 0, 0, 0, tzinfo=UTC)
+        )
     res = dict(TEST_CARTPOSITION_RES)
     res["id"] = cr.pk
     res["item"] = item.pk