work around a django-polymorphic bug that breaks certain cascades

see: #1794

awx/main/models/__init__.py

@@ -3,6 +3,7 @@
 
 # Django
 from django.conf import settings # noqa
+from django.db.models.signals import pre_delete  # noqa
 
 # AWX
 from awx.main.models.base import * # noqa
@@ -58,6 +59,18 @@ User.add_to_class('can_access_with_errors', check_user_access_with_errors)
 User.add_to_class('accessible_objects', user_accessible_objects)
 
 
+def cleanup_created_modified_by(sender, **kwargs):
+    # work around a bug in django-polymorphic that doesn't properly
+    # handle cascades for reverse foreign keys on the polymorphic base model
+    # https://github.com/django-polymorphic/django-polymorphic/issues/229
+    for cls in (UnifiedJobTemplate, UnifiedJob):
+        cls.objects.filter(created_by=kwargs['instance']).update(created_by=None)
+        cls.objects.filter(modified_by=kwargs['instance']).update(modified_by=None)
+
+
+pre_delete.connect(cleanup_created_modified_by, sender=User)
+
+
 @property
 def user_get_organizations(user):
     return Organization.objects.filter(member_role__members=user)