migrations: Fix migration 0574 to handle edge-case DirectMessageGroups.

The Django ORM query would not work correctly for pathological
DirectMessageGroups with no Subscription rows. When the Subquery gave
empty results, the UPDATE would set group_size to null - when the point
of the migration was exactly to make sure this column is always set and
allow making the column non-nullable in 0575.

Such DirectMessageGroups can occur as a result doing .delete() on all
UserProfiles that were in the group - or by doing realm deletion via
either .delete() or `manage.py delete_realm`.

The natural choice for group_size of these objects is 0. The simple SQL
query naturally achieves this result, without needing any special
handling for this case.

zerver/migrations/0574_backfill_directmessagegroup_group_size.py

@@ -1,9 +1,8 @@
 # Generated by Django 5.0.6 on 2024-08-16 06:44
 
-from django.db import migrations, transaction
+from django.db import connection, migrations
 from django.db.backends.base.schema import BaseDatabaseSchemaEditor
 from django.db.migrations.state import StateApps
-from django.db.models import Count, Model, OuterRef, Subquery
 
 BATCH_SIZE = 1000
 
@@ -11,9 +10,7 @@ BATCH_SIZE = 1000
 def backfill_group_size_field_for_direct_message_groups(
     apps: StateApps, schema_editor: BaseDatabaseSchemaEditor
 ) -> None:
-    DirectMessageGroup = apps.get_model("zerver", "DirectMessageGroup")
     Recipient = apps.get_model("zerver", "Recipient")
-    Subscription = apps.get_model("zerver", "Subscription")
 
     RECIPIENT_DIRECT_MESSAGE_GROUP = 3
 
@@ -35,31 +32,35 @@ def backfill_group_size_field_for_direct_message_groups(
 
     while recipient_id_lower_bound <= max_recipient_id:
         do_backfill_group_size_field_for_direct_message_groups(
-            Subscription,
-            DirectMessageGroup,
             recipient_id_lower_bound,
             min(recipient_id_lower_bound + BATCH_SIZE, max_recipient_id),
         )
         recipient_id_lower_bound += BATCH_SIZE + 1
 
 
-@transaction.atomic
 def do_backfill_group_size_field_for_direct_message_groups(
-    subscription_model: type[Model],
-    direct_message_group_model: type[Model],
     recipient_id_lower_bound: int,
     recipient_id_upper_bound: int,
 ) -> None:
-    direct_message_group_sub_size = (
-        subscription_model._default_manager.filter(recipient=OuterRef("recipient"))
-        .values("recipient")
-        .annotate(count=Count("id"))
-        .values("count")[:1]
-    )
+    with connection.cursor() as cursor:
+        update_query = """
+        UPDATE zerver_huddle
+        SET group_size = (
+            SELECT COUNT(*)
+            FROM zerver_subscription
+            WHERE zerver_subscription.recipient_id = zerver_huddle.recipient_id
+        )
+        WHERE zerver_huddle.recipient_id BETWEEN %(lower_bound)s AND %(upper_bound)s
+        AND zerver_huddle.group_size IS NULL
+        """
 
-    direct_message_group_model._default_manager.filter(
-        recipient__id__range=(recipient_id_lower_bound, recipient_id_upper_bound), group_size=None
-    ).update(group_size=Subquery(direct_message_group_sub_size))
+        cursor.execute(
+            update_query,
+            {
+                "lower_bound": recipient_id_lower_bound,
+                "upper_bound": recipient_id_upper_bound,
+            },
+        )
 
 
 class Migration(migrations.Migration):