[{"s":"v1","id":1},{"s":"v2","id":2},{"s":"v3","id":3},{"s":"v4","id":4},]

db.collection.aggregate([{"$group":{"_id":null,"result":{"$addToSet":{"k":"$s","v":"$id"}}}},{"$set":{"result":{"$arrayToObject":"$result"}}},{"$replaceRoot":{"newRoot":"$result"}}])