Extend permission backend with get_queryset(user, model)

Permissions on objects are based on two mechanisms that developers have to implement:

  • returning if a user has a permission on an object instance
  • filtering a queryset based on a user object and eventually a permission name

Currently, permission backend allows developers to implement the first mechanism: you can allow a specific permission on an object with the permission backend.

This works extremely well even for complex use cases: you get an model object, a user, a permission name and you can return True.

Exemple:

    def has_perm(self, user_obj, perm, obj=None):    
        if not user_obj.is_authenticated or not isinstance(obj, SomeModel):    
            return False
    
        return user_obj.is_superuser or obj.related_model_fk in user_obj.related_model_m2m.all()    

However, permission framework should also allow developers to implement the second security mechanism: getting a filtered queryset with objects a user should be able to see, eventually for a given permission. Such implementation could look like:

    def filter_queryset(self, user_obj, perm, queryset=None):
        if not queryset.model == SomeModel:
            return queryset

        if not user_obj.is_authenticated:    
            return queryset.none()    
    
        return queryset.filter(related_model_fk__in=user_obj.related_model_m2m.all())

The admin views could use this, and django.contrib.auth could provide generic views extensions which do check permissions, removing the need to share a mixin that just does return a Mixin with a get_queryset method to complement the code that they have in the permission backend. It would reduce chances to make a mistake when updating permission code if it’s all at the same place, an opinion that I consider suited for a framework like Django.

I consider that the subject of making ModelChoiceFields to be able to benefit from this is out of the scope of this ticket, but I could bring it up for discussion if this feature is implemented (ie. DRF serializers have a “context” variables where the request object is set by default, which allows to do user-based validation: a pretty standard requirement).

Ticket was closed pending discussion: https://code.djangoproject.com/ticket/31093#modify