Database
db={"user":[{_id:"d04d53dc-fb88-433e-a1c5-dd41a68d7655",userName:"XYZ User",age:12}],"square":[{_id:"ef6f6ac2-a08a-4f68-a63c-0b4a70285427",userId:"d04d53dc-fb88-433e-a1c5-dd41a68d7655",side:4,area:16}]}
Query
db.square.aggregate([{"$lookup":{"from":"user","localField":"userId","foreignField":"_id","as":"userDoc"}},{"$set":{"userName":{"$first":"$userDoc.userName"}}},{"$unset":"userDoc"}])