membershipworks: Convert EventExt.duration annotation to a Subquery

should be somewhat less performant, but allows for easier aggregation

membershipworks/models.py

@@ -4,7 +4,7 @@ from datetime import datetime
 import django.core.mail.message
 from django.conf import settings
 from django.db import models
-from django.db.models import Exists, F, OuterRef, Sum
+from django.db.models import Exists, F, OuterRef, Subquery, Sum
 from django.utils import timezone
 
 
@@ -415,7 +415,19 @@ class EventInstructor(models.Model):
 
 class EventExtManager(models.Manager["EventExt"]):
     def get_queryset(self) -> models.QuerySet["EventExt"]:
-        return super().get_queryset().annotate(duration=Sum("meeting_times__duration"))
+        return (
+            super()
+            .get_queryset()
+            .annotate(
+                duration=Subquery(
+                    EventMeetingTime.objects.filter(event=OuterRef("pk"))
+                    .values("event__pk")
+                    .annotate(d=Sum("duration"))
+                    .values("d")[:1],
+                    output_field=models.DurationField(),
+                ),
+            )
+        )
 
 
 class EventExt(Event):