Django model query with custom select fields

Question

I'm using the row-level permission model known as django-granular-permissions (http://code.google.com/p/django-granular-permissions/). The permission model simply has just two more fields which are content-type and object id.

I've used the following query:

 User.objects.filter(Q(row_permission_set__name='staff') | \
     Q(row_permission_set__name='student'), \
     row_permission_set__object_id=labsite.id)

I want to add is_staff and is_student boolean fields to the result set without querying everytime when I fetch the result.

Django documentation shows extra() method of querysets, but I can't figure out what I should write for plain SQL selection query with this relation.

How to do this?

Answer 1

Normally you'd use select_related() for things like this, but unfortunately it doesn't work on reverse relationships. What you could do is turn the query around:

users = [permission.user for permission in Permission.objects.select_related('user').filter(...)]

Answer 2

.extra(select={'is_staff': "%s.name='staff'" % Permission._meta.db_table, 'is_student': "%s.name='student'" % Permission._meta.db_table, })