$lookup nested external id after $group - mongodb

Question

I want to replace the external user ids with the real user properties after $lookup, my data:
"comments" : [
{
"user_Id" : ObjectId("aaa"),
"content" : "aaaa",
"rep" : [
{
"user_Id" : ObjectId("bbb"),
"comment" : "bbbb",
},
{
"user_Id" : ObjectId("ccc"),
"comment" : "cccc",
}
]
},
{
"user_Id" : ObjectId("ddd"),
"content" : "ddd",
"rep" : [ ]
}
]
User collection:
"users" : [
{
"_id" : ObjectId("aaa"),
"name" : "user1",
"email" : "test1#test.com",
},
{
"_id" : ObjectId("bbb"),
"username" : "user2",
"email" : "test2#test.com",
}
]
What i want to archieve:
"comments" : [
{
"user" : {
"_id" : ObjectId("aaa"),
"name" : "user1",
"email" : "test1#test.com",
}
"content" : "aaaa",
"rep" : [
{
"userId" : {
"_id" : ObjectId("bbb"),
"username" : "user2",
"email" : "test2#test.com",
},
"comment" : "bbbb",
},
{
"user" : {
"_id" : ObjectId("aaa"),
"name" : "user1",
"email" : "test1#test.com",
},
"comment" : "cccc",
}
]
},
{
"user" : {
"_id" : ObjectId("bbb"),
"username" : "user2",
"email" : "test2#test.com",
},
"content" : "ddd",
"rep" : [ ]
}
]
Right now i managed to get my user info from the external id but i'm going crazy trying to get the user object inside the replies too, i tried to group after grouping but nothing to do, this is what i did:
db.pages.aggregate([
{
$match: { _id: ObjectId('5db599f3fffdee1c822269e0b3') }
},
{
$project: {
comments: 1,
}
},
{ $unwind: '$comments' },
{
$lookup:
{
from: 'users',
localField: 'comments.user_Id',
foreignField: '_id',
as: 'us'
}
},
{ $unwind: '$us' },
{
$group: {
_id: {
user: {
id: '$us._id',
name: '$us.username',
email: '$us.email',
},
comments: {
comment: '$comments.comment',
rep: '$comments.rep'
},
}
}
}
]).pretty()