[{"userId":"123","friendsArray":[{"lastTimeStamp":1,"message":"1"},{"lastTimeStamp":4,"message":"4"},{"lastTimeStamp":3,"message":"3"},{"lastTimeStamp":2,"message":"2"},{"lastTimeStamp":5,"message":"5"}]}]

db.collection.aggregate([{"$match":{"userId":"123"}},{"$unwind":{"path":"$friendsArray"}},{"$sort":{"friendsArray.lastTimeStamp":1}},{"$limit":10},{"$replaceRoot":{"newRoot":"$friendsArray"}}])