db={"comments":[{"_id":"5a934e000102030405000000","commentText":"text1","arrayOfReplies":[{"replyText":"reply1","replier":"5a934e000102030405000001"},{"replyText":"reply2","replier":"5a934e000102030405000002"}]}],"users":[{"_id":"5a934e000102030405000001","username":"user1","bio":"bio1"},{"_id":"5a934e000102030405000002","username":"user2","bio":"bio2"},{"_id":"5a934e000102030405000003","username":"user3","bio":"bio3"},{"_id":"5a934e000102030405000004","username":"user4","bio":"bio4"}]}

db.comments.aggregate([{$unwind:{"path":"$arrayOfReplies","preserveNullAndEmptyArrays":true}},{$lookup:{from:"users",localField:"arrayOfReplies.replier",foreignField:"_id",as:"replier"}},{$addFields:{"arrayOfReplies.replier":{$arrayElemAt:["$replier",0]}}},{$project:{"replier":0}},{$group:{_id:"$_id","arrayOfReplies":{"$push":"$arrayOfReplies"},commentText:{"$first":"$commentText"}}}])