db={"coll1":[{postId:"232323",byUser:"userid1",post:"This is my first post",commentsOnPost:[{commentId:"232323_8888",byUser:"userid2",comment:"Congrats",repliesOnPost:[]}]}],"coll2":[{userid:"userid1",displayName:"Bob",profilePic:"https://example.com/bob_profile.png"},{userid:"userid2",displayName:"Lorish",profilePic:"https://example.com/lorish_profile.png"}]}

db.coll1.aggregate([{"$lookup":{"from":"coll2","localField":"byUser","foreignField":"userid","as":"byUser"}},{"$unwind":"$byUser"},{"$unwind":"$commentsOnPost"},{"$lookup":{"from":"coll2","localField":"commentsOnPost.byUser","foreignField":"userid","as":"commentsOnPost.byUser"}},{"$unwind":"$commentsOnPost.byUser"},{"$unwind":"$commentsOnPost.repliesOnPost"},{"$lookup":{"from":"coll2","localField":"commentsOnPost.repliesOnPost.byUser","foreignField":"userid","as":"commentsOnPost.repliesOnPost.byUser"}},{"$unwind":"$commentsOnPost.repliesOnPost"},{"$group":{"_id":"$_id",postId:{"$first":"$postId"},post:{"$first":"$post"},"commentsOnPost":{"$push":"$commentsOnPost"}}}])