def collection(self, collection_id):
        """Create a sub-collection underneath the current document.

        Args:
            collection_id (str): The sub-collection identifier (sometimes
                referred to as the "kind").

        Returns:
            ~.firestore_v1beta1.collection.CollectionReference: The
            child collection.
        """
        child_path = self._path + (collection_id,)
        return self._client.collection(*child_path)