db={posts:[{"id":"1","comments":[{_id:"c1",userId:"u1",text:"Hi there, who are you?"},{_id:"c2",userId:"u1",text:"Hi! Are you still there?"},]}],users:[{"_id":"u1","name":"James","username":"jamo"}]}

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