code: def end_before(self, document_fields):
          
          return self._cursor_helper(document_fields, before=True, start=False)
docs: End query results before a particular document value.
  
          The result set will **exclude** the document specified by
          ``document_fields``.
  
          If the current query already has specified an end cursor -- either
          via this method or
          :meth:`~.firestore_v1beta1.query.Query.end_at` -- 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
              "end before" cursor.