How to use the omegaml.store.qops.make_sortkey function in omegaml

def _get_cursor(self):
        projection = make_tuple(self.columns)
        projection += make_tuple(self._get_frame_index())
        if not self.sort_order:
            # implicit sort
            projection += make_tuple(self._get_frame_om_fields())
        cursor = self.collection.find(projection=projection)
        if self.sort_order:
            cursor.sort(qops.make_sortkey(make_tuple(self.sort_order)))
        if self.head_limit:
            cursor.limit(self.head_limit)
        if self.skip_topn:
            cursor.skip(self.skip_topn)
        return cursor

# if the merge field is the same in both frames, we already
                # have it from left
                continue
            if right_col in self.columns:
                left_col = '%s%s' % (right_col, suffixes[1])
            else:
                left_col = '%s' % right_col
            project[left_col] = '$%s.%s' % (target_field, right_col)
        expected_columns = list(project.keys())
        project = {"$project": project}
        # store merged documents and return an MDataFrame to it
        out = qops.OUT(target_name)
        pipeline = [lookup, unwind, project]
        if sort:
            sort_cols = make_list(on or [left_on, right_on])
            sort_key = qops.make_sortkey(sort_cols)
            sort = qops.SORT(**dict(sort_key))
            pipeline.append(sort)
        pipeline.append(out)
        if inspect:
            result = pipeline
        else:
            result = self.collection.aggregate(pipeline)
            result = MDataFrame(self.collection.database[target_name],
                                force_columns=expected_columns)
        return result