models: Manage index from migration 0180 with Django.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/migrations/0180_usermessage_add_active_mobile_push_notification.py

@@ -1,10 +1,12 @@
 # Generated by Django 1.11.14 on 2018-08-01 23:05
 
 import bitfield.models
-from django.db import migrations
+from django.db import migrations, models
+from django.db.models import Q
 
 
 class Migration(migrations.Migration):
+    atomic = False
 
     dependencies = [
         ("zerver", "0179_rename_to_digest_emails_enabled"),
@@ -55,12 +57,13 @@ class Migration(migrations.Migration):
                 default=0,
             ),
         ),
-        migrations.RunSQL(
-            """
-            CREATE INDEX zerver_usermessage_active_mobile_push_notification_id
-                ON zerver_usermessage (user_profile_id, message_id)
-                WHERE (flags & 4096) != 0;
-            """,
-            reverse_sql="DROP INDEX zerver_usermessage_active_mobile_push_notification_id;",
+        migrations.AddIndex(
+            model_name="usermessage",
+            index=models.Index(
+                "user_profile",
+                "message",
+                condition=Q(flags__andnz=4096),
+                name="zerver_usermessage_active_mobile_push_notification_id",
+            ),
         ),
     ]

zerver/models.py

@@ -3125,6 +3125,18 @@ class AbstractUserMessage(models.Model):
 class UserMessage(AbstractUserMessage):
     message: Message = models.ForeignKey(Message, on_delete=CASCADE)
 
+    class Meta(AbstractUserMessage.Meta):
+        indexes = [
+            models.Index(
+                "user_profile",
+                "message",
+                condition=Q(
+                    flags__andnz=AbstractUserMessage.flags.active_mobile_push_notification.mask
+                ),
+                name="zerver_usermessage_active_mobile_push_notification_id",
+            ),
+        ]
+
 
 def get_usermessage_by_message_id(
     user_profile: UserProfile, message_id: int