mongodb order by list

use foo;
db.foo.ensureIndex({bar: 1});
db.foo.save({ bar: 12});
db.foo.save({ bar: 57});
db.foo.save({ bar: 33});
db.foo.save({ bar: 7});
db.foo.save({ bar: 101});
db.foo.find({});
/*
{ "_id" : ObjectId("4e0a0e4e708f107ecdf9aae0"), "bar" : 12 }
{ "_id" : ObjectId("4e0a0e52708f107ecdf9aae1"), "bar" : 57 }
{ "_id" : ObjectId("4e0a0e55708f107ecdf9aae2"), "bar" : 33 }
{ "_id" : ObjectId("4e0a0e5a708f107ecdf9aae3"), "bar" : 7 }
{ "_id" : ObjectId("4e0a0e5d708f107ecdf9aae4"), "bar" : 101 }
*/

/* a wild guess at querying/sorting by a list syntax. fails. */
db.foo.find({bar : { $in: [101, 7, 12, 57] } }).sort({bar: [101, 7, 12, 57]});
/*
{ "_id" : ObjectId("4e0a0e5a708f107ecdf9aae3"), "bar" : 7 }
{ "_id" : ObjectId("4e0a0e4e708f107ecdf9aae0"), "bar" : 12 }
{ "_id" : ObjectId("4e0a0e52708f107ecdf9aae1"), "bar" : 57 }
{ "_id" : ObjectId("4e0a0e5d708f107ecdf9aae4"), "bar" : 101 }
*/

/* ... and querying/sorting by ObjectId would be really nice too */
db.foo.find({_id :
{ $in: [
    ObjectId("4e0a0e5d708f107ecdf9aae4"), /* 101 */
    ObjectId("4e0a0e5a708f107ecdf9aae3"), /* 7 */
    ObjectId("4e0a0e4e708f107ecdf9aae0"), /* 12 */
    ObjectId("4e0a0e52708f107ecdf9aae1")] /* 57 */
}}).sort(
{ _id: [
    ObjectId("4e0a0e5d708f107ecdf9aae4"), /* 101 */
    ObjectId("4e0a0e5a708f107ecdf9aae3"), /* 7 */
    ObjectId("4e0a0e4e708f107ecdf9aae0"), /* 12 */
    ObjectId("4e0a0e52708f107ecdf9aae1")] /* 57 */
});
/*
{ "_id" : ObjectId("4e0a0e5a708f107ecdf9aae3"), "bar" : 7 }
{ "_id" : ObjectId("4e0a0e4e708f107ecdf9aae0"), "bar" : 12 }
{ "_id" : ObjectId("4e0a0e52708f107ecdf9aae1"), "bar" : 57 }
{ "_id" : ObjectId("4e0a0e5d708f107ecdf9aae4"), "bar" : 101 }
*/