Get a mapfile owned by a user from the database by
            map_id.

def _get_map_from_user_by_id(self, user, map_id):
        """ Get a mapfile owned by a user from the database by
            map_id. """
        req = Session.query(Map).select_from(join(Map, User))
        try:
            return req.filter(and_(User.login==user, Map.id==map_id)).one()
        except Exception, e:
            return None