db={"user_relations":[{"_id":"someRandomString","userId":"userId1","friendsArray":[{"userId":"userId2","lastTimestamp":19236752642,"message":"Hellooo"},{"userId":"userId3","lastTimestamp":12236752342,"message":"Yeah"}]}],"users":[{"_id":"userId2","nameAndSurname":"Name 2","arrayOfImages":["wwww.urlToImage2.jpeg"],"favouritePlayer":"Monfils2","sport":"Tennis2"},{"_id":"userId3","nameAndSurname":"Name 3","arrayOfImages":["wwww.urlToImage3.jpeg"],"favouritePlayer":"Monfils3","sport":"Tennis3"}]}

db.user_relations.aggregate([{"$match":{"$expr":{"$eq":["$userId","userId1"]}}},{"$unwind":{"path":"$friendsArray"}},{"$sort":{"friendsArray.lastTimeStamp":1}},{"$limit":10},{"$replaceRoot":{"newRoot":"$friendsArray"}},{"$lookup":{"from":"users","localField":"userId","foreignField":"_id","as":"joined__"}},{"$unwind":{"path":"$joined__"}},{"$replaceRoot":{"newRoot":{"$mergeObjects":["$joined__","$$ROOT"]}}},{"$project":{"joined__":0}}])