fix: add unique constraint on conference to prevent duplicate split (#21)

* fix: add unique constraint on conference (conference_id, app) to prevent duplicates

When two participants join the same session simultaneously, both call
POST /v1/initialize with the same conferenceId. Without a unique
constraint, both get_or_create calls create separate conference records
(race condition). This splits participants across different conference
pages.

Fix:
- Migration 0002: merges existing duplicates (keeps oldest, moves all
  related events/sessions/connections/issues to it)
- Migration 0003: adds unique_together on (conference_id, app)

After this, concurrent get_or_create calls will correctly find the
existing conference instead of creating duplicates.

* fix: address review feedback on conference unique constraint migration

- Moved imports to top of file/function
- Wrapped each duplicate group in transaction.atomic()
- Added Summary model to reassignment (OneToOne FK to Conference)
- Added safety check on dup.delete() to catch unexpected cascades

* fix: address P1 and P2 review feedback on migration

P1: Removed over-strict cascade-delete guard. Deleting a duplicate
conference normally cascades to M2M join table rows (participant
through table), which is expected. The guard was raising on this.

P2: Replaced hasattr(keeper, 'summary') with a local boolean flag
(keeper_has_summary) that updates after the first summary is moved.
Avoids stale cached reverse OneToOne lookups across iterations.

Author: Boanerges1996

app/migrations/0002_conference_unique_conference_id_app.py

@@ -0,0 +1,65 @@
+# Generated by Django 2.2.28 on 2026-04-09 13:49
+
+from django.db import migrations
+from django.db.models import Count, Min
+
+
+def merge_duplicate_conferences(apps, schema_editor):
+    """
+    Before adding the unique constraint, merge duplicate conferences
+    (same conference_id + app_id). Keep the oldest one, move all
+    related data to it, and delete the rest.
+    """
+    from django.db import transaction
+
+    Conference = apps.get_model('app', 'Conference')
+    GenericEvent = apps.get_model('app', 'GenericEvent')
+    Connection = apps.get_model('app', 'Connection')
+    Session = apps.get_model('app', 'Session')
+    Issue = apps.get_model('app', 'Issue')
+    Summary = apps.get_model('app', 'Summary')
+
+    dupes = (Conference.objects
+        .values('conference_id', 'app_id')
+        .annotate(count=Count('id'), earliest=Min('created_at'))
+        .filter(count__gt=1))
+
+    for dupe in dupes:
+        with transaction.atomic():
+            conferences = Conference.objects.filter(
+                conference_id=dupe['conference_id'],
+                app_id=dupe['app_id'],
+            ).order_by('created_at')
+
+            keeper = conferences.first()
+            duplicates = conferences.exclude(pk=keeper.pk)
+            keeper_has_summary = Summary.objects.filter(conference=keeper).exists()
+
+            for dup in duplicates:
+                GenericEvent.objects.filter(conference=dup).update(conference=keeper)
+                Connection.objects.filter(conference=dup).update(conference=keeper)
+                Session.objects.filter(conference=dup).update(conference=keeper)
+                Issue.objects.filter(conference=dup).update(conference=keeper)
+
+                # Summary is OneToOne — keep the first one, delete the rest
+                if not keeper_has_summary:
+                    Summary.objects.filter(conference=dup).update(conference=keeper)
+                    keeper_has_summary = True
+                else:
+                    Summary.objects.filter(conference=dup).delete()
+
+                for participant in dup.participants.all():
+                    keeper.participants.add(participant)
+
+                dup.delete()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('app', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.RunPython(merge_duplicate_conferences, migrations.RunPython.noop),
+    ]

app/migrations/0003_conference_unique_together.py

@@ -0,0 +1,17 @@
+# Generated by Django 2.2.28 on 2026-04-09 13:49
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('app', '0002_conference_unique_conference_id_app'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='conference',
+            unique_together={('conference_id', 'app')},
+        ),
+    ]

app/models/conference.py

@@ -37,6 +37,7 @@ class Conference(BaseModel):
 
     class Meta:
         db_table = 'conference'
+        unique_together = (('conference_id', 'app'),)
 
     cache_keys = (
         sorted(('id',)),