Unfortunately, according to the Django ticket for this 
(https://code.djangoproject.com/ticket/13914) it seems that we aren't going 
to get this functionality built-in any time soon. However, after many hours 
of inspection in the Django source, I think I have found a solution...

1. In Django 1.3 the User model already has it's own UserManager, so we 
must update that instead of overwriting it. This makes adding natural keys 
for Users much easier.
2. The ValidationError is because it's not even trying to use the natural 
keys- this code uses private fields like _default_manager, which were 
initialised before the manager was replaced. In the end, since we are only 
adding a method and nothing else, the most appropriate way to fix this is 
to change the __class__ of the objects member in Group.
3. I don't appear to need add_to_class(), but YMMV. An example would be 
good.

def uget_by_natural_key(self, username):
    return self.get(username=username)
def gnatural_key(self):
        return self.name
def unatural_key(self):
        return self.username
class GroupManager(models.Manager):
    def get_by_natural_key(self, name):
        return self.get(name=name)

UserManager.get_by_natural_key = uget_by_natural_key
Group.objects.__class__ = GroupManager
User.natural_key = unatural_key
Group.natural_key = gnatural_key

For reference, I put this code in models.py.

Disclaimer: This kind of hackery may have serious side-effects, especially 
since Django has done a lot of initialisation by the time we get to this 
point.

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