Mongo: sort documents by array of ids

Say you have an array of document ids stored elsewhere (e.g. in Redis sorted set). What is the most efficient way of querying Mongo documents with `{ _id: { $in: ids }}` and having the results sorted in the same ordering as it was in `ids` array? Example ``` var ids = [3,2,1,6,7]; db.records.find({ _id: { $in: ids }}) .sort({ // ??? }); // expecting [{_id: 3}, {_id: 2}, {_id: 1}, {_id: 6}, {_id: 7}] ``` P.S. I know I can do the sorting in application, but I wonder, if it could be done with more efficiency at the backend.