Rewrite migration for Django strangeness

aleksis/apps/paweljong/migrations/0014_move_sent_to_to_through.py

@@ -12,12 +12,14 @@ def migrate_to_new_through(apps, schema_editor):
 
     db_alias = schema_editor.connection.alias
 
-    for event in Event.objects.using(db_alias).iterator():
-        for info_mailing in event.info_mailings.all():
-            through = Through.objects.create(event=event, info_mailing=info_mailing)
-            for person in info_mailing.sent_to.all():
-                if EventRegistration.objects.filter(person_id=person.id, event_id=event.id).exists():
-                    through.sent_to.add(person)
+    for through_old in Event.info_mailings.through.objects.all():
+        event = through_old.event
+        info_mailing = through_old.infomailing
+
+        through = Through.objects.create(event=event, info_mailing=info_mailing)
+        for person in info_mailing.sent_to.all():
+            if EventRegistration.objects.filter(person_id=person.id, event_id=event.id).exists():
+                through.sent_to.add(person)
 
 
 class Migration(migrations.Migration):