Create a document in the Firestore database with the provided data.

        Args:
            document_data (dict): Property names and values to use for
                creating the document.
            document_id (Optional[str]): The document identifier within the
                current collection. If not provided, an ID will be
                automatically assigned by the server (the assigned ID will be
                a random 20 character string composed of digits,
                uppercase and lowercase letters).

        Returns:
            Tuple[google.protobuf.timestamp_pb2.Timestamp, \
                ~.firestore_v1beta1.document.DocumentReference]: Pair of

            * The ``update_time`` when the document was created (or
              overwritten).
            * A document reference for the created document.

        Raises:
            ~google.cloud.exceptions.Conflict: If ``document_id`` is provided
                and the document already exists.

def add(self, document_data, document_id=None):
        """Create a document in the Firestore database with the provided data.

        Args:
            document_data (dict): Property names and values to use for
                creating the document.
            document_id (Optional[str]): The document identifier within the
                current collection. If not provided, an ID will be
                automatically assigned by the server (the assigned ID will be
                a random 20 character string composed of digits,
                uppercase and lowercase letters).

        Returns:
            Tuple[google.protobuf.timestamp_pb2.Timestamp, \
                ~.firestore_v1beta1.document.DocumentReference]: Pair of

            * The ``update_time`` when the document was created (or
              overwritten).
            * A document reference for the created document.

        Raises:
            ~google.cloud.exceptions.Conflict: If ``document_id`` is provided
                and the document already exists.
        """
        if document_id is None:
            parent_path, expected_prefix = self._parent_info()

            document_pb = document_pb2.Document()

            created_document_pb = self._client._firestore_api.create_document(
                parent_path,
                collection_id=self.id,
                document_id=None,
                document=document_pb,
                mask=None,
                metadata=self._client._rpc_metadata,
            )

            new_document_id = _helpers.get_doc_id(created_document_pb, expected_prefix)
            document_ref = self.document(new_document_id)
            set_result = document_ref.set(document_data)
            return set_result.update_time, document_ref
        else:
            document_ref = self.document(document_id)
            write_result = document_ref.create(document_data)
            return write_result.update_time, document_ref