code: def add(self, relation):
          
          r = Relation(relation)
          key = r.relation_key[0]
          if key not in self.relations:
              self.relations[key] = r
              self.relations[r.relation_key[1]]= r
docs: relation is a string list, just like:
          ['User.id = Group.user', 'User.username = Group.username']
          :param relation:
          :return: