code: def _get_map_from_user_by_id(self, user, 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
docs: Get a mapfile owned by a user from the database by
              map_id.