Add the relation inner to the cache, under the schema schema and
        identifier identifier

        :param BaseRelation relation: The underlying relation.

def add(self, relation):
        """Add the relation inner to the cache, under the schema schema and
        identifier identifier

        :param BaseRelation relation: The underlying 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()))
        )