Understanding behavior of django model save

Question

When using the shell, this confused me (the permissions did not reflect changes):

>>> user = User.objects.get(username='test')
>>> user.get_all_permissions()
set([])
>>> p = Permission.objects.get(codename="add_slide")
>>> user.user_permissions.add(p)
>>> user.save()
>>> user.get_all_permissions()
set([])
>>> user = User.objects.get(username='test')
>>> user.get_all_permissions()
set([u'slides.add_slide'])

Why did the user object not update on save?
Is there a way save and update the object?

Answer 1

Django doesn't update in-memory objects when on-disk objects change. Your first user still looks as it does when it was read from disk.

Answer 2

This is happening because User has a many2many relationship with Permission, so when you do user.user_permissions.add(p), the "auth_user" table does not get updated. Instead, its the "through" table for that relationship ("auth_user_user_permissions") that gets updated. In fact, you don't need to call user.save() at all.

The get_all_permissions() method appears to be using cached data, so if you want the latest changes, use:

user.user_permissions.all()

Note that this will return a list of Permission objects, so if you want a list of codenames in the exact same format as a get_all_permissions() call (not sure why you would though), you can do this:

set(user.user_permissions.values_list('codename', flat=True))