API: Fix old meta values being returned when updating event (fixes #5077)

src/pretix/api/serializers/event.py

@@ -378,6 +378,8 @@ class EventSerializer(SalesChannelMigrationMixin, I18nAwareModelSerializer):
                     if prop.name not in meta_data:
                         current_object.delete()
 
+            instance._prefetched_objects_cache.clear()
+
         # Item Meta properties
         if item_meta_properties is not None:
             current = list(event.item_meta_properties.all())
@@ -398,6 +400,8 @@ class EventSerializer(SalesChannelMigrationMixin, I18nAwareModelSerializer):
                 if prop.name not in list(item_meta_properties.keys()):
                     prop.delete()
 
+            instance._prefetched_objects_cache.clear()
+
         # Seats
         if seat_category_mapping is not None or ('seating_plan' in validated_data and validated_data['seating_plan'] is None):
             current_mappings = {

src/tests/api/test_events.py

@@ -734,6 +734,9 @@ def test_event_update(token_client, organizer, event, item, meta_prop):
         format='json'
     )
     assert resp.status_code == 200
+    assert resp.data["meta_data"] == {
+        meta_prop.name: "Workshop"
+    }
     with scopes_disabled():
         assert organizer.events.get(slug=resp.data['slug']).meta_values.filter(
             property__name=meta_prop.name, value="Workshop"