User group assignment track in django admin

Question

In Django I want to track how the groups are assigned to User, right now we are assigning group to user through Django Admin. I tried to use m2m_changed signal on User.groups.through table but In Django admin when group assigned to User then it first clears out existing groups of user and add updated groups so it creates unnecessary call to "pre_clear" and "post_clear" action of signal so because of it signal is not working in all cases.

Can anyone suggest me the way to override Django User save method and handle changed group in that method? or any other way to track user group assignment?

I am using Django 1.6

Answer 1

That's a known problem of Django and it was fixed only in 1.9: https://docs.djangoproject.com/en/1.9/releases/1.9/#related-set-direct-assignment.

Direct assignment of related objects in the ORM used to perform a clear() followed by a call to add(). This caused needlessly large data changes and prevented using the m2m_changed signal to track individual changes in many-to-many relations.

Direct assignment now relies on the the new set() method on related managers which by default only processes changes between the existing related set and the one that’s newly assigned. The previous behavior can be restored by replacing direct assignment by a call to set() with the keyword argument clear=True.

ModelForm, and therefore ModelAdmin, internally rely on direct assignment for many-to-many relations and as a consequence now use the new behavior.

If you were manually using user.groups.add(foo) or user.groups.remove(foo) then your current signal receiver should work fine. But django admin uses assignment so it causes clear and set on every save from django-admin. To fix this problem without updating to django 1.9 you have to process both pre_clear and post_add: store cleared pks to some "special" attribute of instance in preclear and then calculate what was added or removed.

def pre_clear_handler(sender, *args, **kwargs):
    # ...
    instance._cleared_groups_pk_set = set([group.pk for group in instance.groups.all()])  # or `set(instance.groups.all().values_list('pk', flat=True))
    tracker = SomeTrackingModel()
    tracker.removed_group_ids = instance._cleared_groups_pk_set
    tracker.save()  # post_add may not be called if groups are fully cleared
    instance._changes_tracker_helper = tracker

def post_add_handler(sender, *args, **kwargs):
    # ...
    prev_groups_pk_set = getattr(instance, '_cleared_groups_pk_set', None)
    if prev_groups_pk_set is not None:
        not_realy_added_pk_set = kwargs['pk_set']
        removed_pks = prev_groups_pk_set - not_realy_added_pk_set
        realy_added_pks = not_realy_added_pk_set - prev_groups_pk_set
    else:
        removed_pks = set()
        realy_added_pks = kwargs['pk_set']
    tracker = getattr(instance, '_changes_tracker_helper', SomeTrackingModel())
    tracker.removed_group_ids = removed_pks
    tracker.added_group_ids = realy_added_pks
    tracker.save()