[{userName:"test1",score:14},{userName:"test2",score:2},{userName:"test3",score:1},{userName:"test4",score:0}]

db.collection.aggregate([{$sort:{score:-1}},{$project:{_id:0,res:[{k:"$userName",v:"$score"}]}},{$set:{res:{$arrayToObject:["$res"]}}},{$replaceRoot:{newRoot:"$res"}}])