def document(self, document_id=None):
        """Create a sub-document underneath the current collection.

        Args:
            document_id (Optional[str]): The document identifier
                within the current collection. If not provided, will default
                to a random 20 character string composed of digits,
                uppercase and lowercase and letters.

        Returns:
            ~.firestore_v1beta1.document.DocumentReference: The child
            document.
        """
        if document_id is None:
            document_id = _auto_id()

        child_path = self._path + (document_id,)
        return self._client.document(*child_path)