fixes migration 0037_remove_group_messaging.py to work in SQLite

Author: evgenyfadeev

askbot/migrations/0037_remove_group_messaging.py

@@ -1,28 +1,48 @@
 """Forward-only teardown of askbot.deps.group_messaging.
 
-Postgres-targeted: relies on `DROP TABLE IF EXISTS ... CASCADE`. Also
-removes the dep's `django_migrations` rows so Django doesn't keep
-referencing an app that no longer exists.
+Drops the five model tables plus the two auto-created M2M through tables,
+in child-first order so plain DROP TABLE works on SQLite and MySQL.
+On Postgres we append CASCADE as a safety net in case a deployment has
+added its own FKs into these tables.
 
-For a MySQL operator: drop the `CASCADE` keyword from each DROP (MySQL
-behaviour differs; askbot has no FKs into these tables, so plain
-`DROP TABLE IF EXISTS` is sufficient). For SQLite: same — drop `CASCADE`.
+Also removes the dep's django_migrations rows so Django stops referencing
+an app that no longer exists.
 """
 from django.db import migrations
 
 
+TABLES_IN_DROP_ORDER = [
+    # M2M through tables first — they reference Message / SenderList.
+    'group_messaging_message_recipients',
+    'group_messaging_senderlist_senders',
+    # Tables with FKs into Message.
+    'group_messaging_lastvisittime',
+    'group_messaging_messagememo',
+    # SenderList is independent of Message.
+    'group_messaging_senderlist',
+    # Message has a self-FK; safe to drop once the children above are gone.
+    'group_messaging_message',
+    # Independent of the rest.
+    'group_messaging_unreadinboxcounter',
+]
+
+
+def drop_group_messaging_tables(apps, schema_editor):
+    vendor = schema_editor.connection.vendor
+    suffix = ' CASCADE' if vendor == 'postgresql' else ''
+    with schema_editor.connection.cursor() as cursor:
+        for table in TABLES_IN_DROP_ORDER:
+            cursor.execute(f'DROP TABLE IF EXISTS {table}{suffix};')
+        cursor.execute(
+            "DELETE FROM django_migrations WHERE app='group_messaging';"
+        )
+
+
 class Migration(migrations.Migration):
     dependencies = [('askbot', '0036_migrate_email_validation_required')]
     operations = [
-        migrations.RunSQL(
-            sql=[
-                "DROP TABLE IF EXISTS group_messaging_message CASCADE;",
-                "DROP TABLE IF EXISTS group_messaging_messagememo CASCADE;",
-                "DROP TABLE IF EXISTS group_messaging_senderlist CASCADE;",
-                "DROP TABLE IF EXISTS group_messaging_lastvisittime CASCADE;",
-                "DROP TABLE IF EXISTS group_messaging_unreadinboxcounter CASCADE;",
-                "DELETE FROM django_migrations WHERE app='group_messaging';",
-            ],
-            reverse_sql=migrations.RunSQL.noop,
+        migrations.RunPython(
+            drop_group_messaging_tables,
+            reverse_code=migrations.RunPython.noop,
         ),
     ]