code: def relation(self, attr):
          
          value = self.get(attr)
          if value is not None:
              if not isinstance(value, leancloud.Relation):
                  raise TypeError('field %s is not Relation'.format(attr))
              value._ensure_parent_and_key(self, attr)
              return value
          return leancloud.Relation(self, attr)
docs: 返回对象上相应字段的 Relation
  
          :param attr: 字段名
          :type attr: string_types
          :return: Relation
          :rtype: leancloud.Relation