db={users:[{"_id":ObjectId("5ee4e727d04e4b4ac1ef115b"),"name":"Ashutosh Tiwari","age":21,"email":"ashutosh@gmail.com"},{"_id":ObjectId("5ee4e727d04e4b4ac1ef115c"),"name":"Maximilian","age":32,"email":"max@yahoo.com"}],posts:[{"_id":ObjectId("5ee51b7ed9f661cad505fcc6"),"title":"First One","text":"Hey this is the first Author","author":ObjectId("5ee4e727d04e4b4ac1ef115c"),"comments":[{"user":ObjectId("5ee4e727d04e4b4ac1ef115b"),"comment":"This is my comment from Ashutosh"},{"user":ObjectId("5ee4e727d04e4b4ac1ef115c"),"comment":"This is my comment from Maximilian"}]},{"_id":ObjectId("5ee5353cd9f661cad505fcc8"),"title":"First One","author":ObjectId("5ee4e727d04e4b4ac1ef115c"),"comments":[{"user":ObjectId("5ee4e727d04e4b4ac1ef115b"),"comment":"This is my comment"}]}]}

db.posts.aggregate([{$unwind:"$comments"},{$lookup:{from:"users",localField:"comments.user",foreignField:"_id",as:"comments.user"}},{$unwind:"$comments.user"},{$group:{_id:"$_id",author:{$first:"$author"},text:{$first:"$text"},title:{$first:"$title"},comments:{$push:"$comments"}}}])