class UserProfile(User):
    middle_name = models.CharField(max_length=30, blank=True)

    def save(self, force_insert=False, force_update=False):
        grp = Group.objects.get(name='manager')
        self.groups = grp
        
        super(UserProfile, self).save(force_insert, force_update)

ValueError at /admin/registration/userprofile/add/
'UserProfile' instance needs to have a primary key value before a many-to-many relationship can be used.Request Method:	POST
Request URL:	http://127.0.0.1:8000/admin/registration/userprofile/add/
Exception Type:	ValueError
Exception Value:	'UserProfile' instance needs to have a primary key value before a many-to-many relationship can be used.
Exception Location:	/usr/lib/python2.5/site-packages/django/db/models/fields/related.py in __init__, line 380

class UserProfile (User):
    middle_name = models.CharField(max_length=30, blank=True)

    def save(self, force_insert=False, force_update=False):
        super(UserProfile, self).save(force_insert, force_update)
        grp = Group.objects.get(name='manager')
        self.groups.add(grp)
        super(UserProfile, self).save(force_insert, force_update)