gistfile1.txt

Index:
        {
                "v" : 1,
                "key" : {
                        "arid" : 1,
                        "_id" : -1
                },
                "ns" : "feeds.artistEvents",
                "name" : "arid_1__id_-1",
                "background" : true
        },


Uses the index:
db.artistEvents.find({ arid: 1239926 }).sort({ _id: -1 }).explain()

Doesn't use the index:
db.artistEvents.find({ $query: { arid: 1239926 }, $orderby: { _id: -1 } }).explain()