class MyModelBackend(ModelBackend):
    def authenticate(self, username=None, password=None):
    if funcao_que_loga_no_banco(username, password):
            user, new = User.objects.get_or_create(username=username)
            if new:
                user.set_unusable_password()
                user.save()
        else:
            user = None
        return user