code: def _setdefault(self, relation):
          
          self.add_schema(relation.database, relation.schema)
          key = relation.key()
          return self.relations.setdefault(key, relation)
docs: Add a relation to the cache, or return it if it already exists.
  
          :param _CachedRelation relation: The relation to set or get.
          :return _CachedRelation: The relation stored under the given relation's
              key