def _only_mapper_zero(self, rationale=None):
    if (len(self._entities) > 1):
        raise sa_exc.InvalidRequestError((rationale or 'This operation requires a Query against a single mapper.'))
    return self._mapper_zero()
def _only_mapper_zero(self, rationale=None):
    if (len(self._entities) > 1):
        raise sa_exc.InvalidRequestError((rationale or 'This operation requires a Query against a single mapper.'))
    return self._mapper_zero()