Fix automatic setting of owner

aleksis/core/schema/custom_event.py

@@ -51,9 +51,9 @@ class CustomEventCreateMutation(DjangoCreateMutation):
         return cls._meta.permissions
 
     @classmethod
-    def before_save(cls, root, info, input, obj: CustomEvent):  # noqa
-        obj.owner = info.context.user.person
-        return obj
+    def before_mutate(cls, root, info, input):  # noqa
+        input["owner"] = info.context.user.person.id
+        return input
 
     @classmethod
     def handle_datetime_start(cls, value, name, info):