code: def add(self, relation):
          
          cached = _CachedRelation(relation)
          logger.debug('Adding relation: {!s}'.format(cached))
          logger.debug('before adding: {}'.format(
              pprint.pformat(self.dump_graph()))
          )
          with self.lock:
              self._setdefault(cached)
          logger.debug('after adding: {}'.format(
              pprint.pformat(self.dump_graph()))
          )
docs: Add the relation inner to the cache, under the schema schema and
          identifier identifier
  
          :param BaseRelation relation: The underlying relation.