def start_at(self, document_fields):
        """Start query results at a particular document value.

        The result set will **include** the document specified by
        ``document_fields``.

        If the current query already has specified a start cursor -- either
        via this method or
        :meth:`~.firestore_v1beta1.query.Query.start_after` -- this will
        overwrite it.

        When the query is sent to the server, the ``document_fields`` will
        be used in the order given by fields set by
        :meth:`~.firestore_v1beta1.query.Query.order_by`.

        Args:
            document_fields (Union[~.firestore_v1beta1.\
                document.DocumentSnapshot, dict, list, tuple]): a document
                snapshot or a dictionary/list/tuple of fields representing a
                query results cursor. A cursor is a collection of values that
                represent a position in a query result set.

        Returns:
            ~.firestore_v1beta1.query.Query: A query with cursor. Acts as
            a copy of the current query, modified with the newly added
            "start at" cursor.
        """
        return self._cursor_helper(document_fields, before=True, start=True)
Start query results at a particular document value.

        The result set will **include** the document specified by
        ``document_fields``.

        If the current query already has specified a start cursor -- either
        via this method or
        :meth:`~.firestore_v1beta1.query.Query.start_after` -- this will
        overwrite it.

        When the query is sent to the server, the ``document_fields`` will
        be used in the order given by fields set by
        :meth:`~.firestore_v1beta1.query.Query.order_by`.

        Args:
            document_fields (Union[~.firestore_v1beta1.\
                document.DocumentSnapshot, dict, list, tuple]): a document
                snapshot or a dictionary/list/tuple of fields representing a
                query results cursor. A cursor is a collection of values that
                represent a position in a query result set.

        Returns:
            ~.firestore_v1beta1.query.Query: A query with cursor. Acts as
            a copy of the current query, modified with the newly added
            "start at" cursor.