Convert the current query into the equivalent protobuf.

        Returns:
            google.cloud.firestore_v1beta1.types.StructuredQuery: The
            query protobuf.

def _to_protobuf(self):
        """Convert the current query into the equivalent protobuf.

        Returns:
            google.cloud.firestore_v1beta1.types.StructuredQuery: The
            query protobuf.
        """
        projection = self._normalize_projection(self._projection)
        orders = self._normalize_orders()
        start_at = self._normalize_cursor(self._start_at, orders)
        end_at = self._normalize_cursor(self._end_at, orders)

        query_kwargs = {
            "select": projection,
            "from": [
                query_pb2.StructuredQuery.CollectionSelector(
                    collection_id=self._parent.id
                )
            ],
            "where": self._filters_pb(),
            "order_by": orders,
            "start_at": _cursor_pb(start_at),
            "end_at": _cursor_pb(end_at),
        }
        if self._offset is not None:
            query_kwargs["offset"] = self._offset
        if self._limit is not None:
            query_kwargs["limit"] = wrappers_pb2.Int32Value(value=self._limit)

        return query_pb2.StructuredQuery(**query_kwargs)