Return user object.
        :param user_id: primary key of user object
        :return: user object

def get_user(self, user_id):
        """
        Return user object.
        :param user_id: primary key of user object
        :return: user object
        """
        user_model = get_user_model()
        try:
            return user_model.objects.get(id=user_id)
        except user_model.DoesNotExist:
            return None