feat(follows): delete object-orphaned Follows...

... based on Django pre_delete signal

actstream/apps.py

@@ -4,6 +4,7 @@
 from django.apps import apps
 from django.apps import AppConfig
 from django.conf import settings
+from django.db.models.signals import pre_delete
 
 from actstream import settings as actstream_settings
 from actstream.signals import action
@@ -25,3 +26,6 @@ def ready(self):
                 DataField(blank=True, null=True).contribute_to_class(
                     action_class, 'data'
                 )
+
+        from actstream.follows import delete_orphaned_follows
+        pre_delete.connect(delete_orphaned_follows)

actstream/follows.py

@@ -0,0 +1,16 @@
+from django.core.exceptions import ImproperlyConfigured
+
+from actstream.models import Follow
+
+
+def delete_orphaned_follows(sender, instance=None, **kwargs):
+    """
+    Clean up Follow objects that refer to a Django object that's being deleted
+    """
+    if str(sender._meta) == 'migrations.migration':
+        return
+
+    try:
+        Follow.objects.for_object(instance).delete()
+    except ImproperlyConfigured:  # raised by actstream for irrelevant models
+        pass

actstream/tests/test_activity.py

@@ -144,7 +144,10 @@ def test_following_models_OR_query(self):
     def test_y_no_orphaned_follows(self):
         follows = Follow.objects.count()
         self.user2.delete()
-        self.assertEqual(follows - 1, Follow.objects.count())
+        # 2 Follow objects are deleted:
+        # * "User2 follows group" because of the on_delete=models.CASCADE
+        # * "User1 follows User2" because of the pre_delete signal
+        self.assertEqual(follows - 2, Follow.objects.count())
 
     def test_z_no_orphaned_actions(self):
         actions = self.user1.actor_actions.count()