Replace complicated query with two queries and process Python-side

website/events/services.py

@@ -272,11 +272,10 @@ def is_organiser(member, event):
             return True
 
         if event:
-            return (
-                member.get_member_groups()
-                .filter(pk__in=event.organisers.values_list("pk"))
-                .exists()
-            )
+            organisers = set(event.organisers.all())
+            member_groups = set(member.get_member_groups())
+
+            return len(organisers.intersection(member_groups)) > 0
 
     return False