> db.test.ensureIndex({a:1})
> db.test.insert({a:1})
> db.test.find({a:1}, {b: {$meta: "indexKey"}})
{  }
> db.test.find({a:1}, {b: {$meta: "indexKey"}}).explain()
{
        "cursor" : "BtreeCursor a_1",
        "isMultiKey" : false,
        "n" : 1,
        "nscannedObjects" : 0,
        "nscanned" : 1,
        "nscannedObjectsAllPlans" : 0,
        "nscannedAllPlans" : 1,
        "scanAndOrder" : false,
        "indexOnly" : true,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "millis" : 0,
        "indexBounds" : {
                "a" : [
                        [
                                1,
                                1
                        ]
                ]
        },
        "server" : "Andrew-Emil-MacBook-Pro.local:27017"
}