db={users:[{"_id":ObjectId("619e13eeabdf5bf20384b74a"),"firstName":"Jane","lastName":"Doe","email":"dnjkwndjkew@gmail.com",},{"_id":ObjectId("619e13eeabdf5bf20384b74b"),"firstName":"Bob","lastName":"Carr","email":"bobcarr@gmail.com",}],notes:[{"_id":ObjectId("629e0a84e0229e4fba0de395"),"order":ObjectId("6298b2a563edd8c9cfa47ba2"),"notes":"Aut pariatur Volupt","status":"pending","createdBy":[{"firstName":"Jim","lastName":"Jones","email":"xxxx7@gmail.com",}],"replies":[{"_id":ObjectId("62a000865f24e1887dfd169e"),"order":ObjectId("6298b2a563edd8c9cfa47ba2"),"noteParent":ObjectId("629e0a84e0229e4fba0de395"),"notes":"Yes this one is ok","createdBy":ObjectId("619e13eeabdf5bf20384b74a"),},{"_id":ObjectId("62a004d1a3c340725d0c732b"),"order":ObjectId("6298b2a563edd8c9cfa47ba2"),"noteParent":ObjectId("629e0a84e0229e4fba0de395"),"notes":"this one too","createdBy":ObjectId("619e13eeabdf5bf20384b74b"),}]}]}

db.notes.aggregate([{"$lookup":{"from":"users","localField":"replies.createdBy","foreignField":"_id","as":"repliesNotes"}},{$set:{"replies":{$map:{input:"$replies",in:{$mergeObjects:["$$this",{createdBy:{$arrayElemAt:["$repliesNotes",{$indexOfArray:["$repliesNotes._id","$$this.id"]}]}}]}}},repliesNotes:"$$REMOVE"}}])