def _get_roles_query(self, session, identifier):
        """
        :type identifier: string
        """
        return (session.query(Role).
                join(role_membership_table, Role.pk_id == role_membership_table.c.role_id).
                join(User, role_membership_table.c.user_id == User.pk_id).
                filter(User.identifier == identifier))
:type identifier: string