how to transform data with 3 collection and $looup - mongodb

Question

I have three collection wanted to fetch records bases on their reference id, I got the result but wanted to transform the data how to make it.
db.post.aggregate([
{
$lookup: {
from: "users",
localField: "created_by",
foreignField: "_id",
as: "users"
}
},
{
$lookup: {
from: "comments",
let: {
p_id: "$_id"
},
pipeline: [
{
$match: {
$expr: {
$eq: [
"$post_id",
"$$p_id"
]
}
}
}
],
as: "comments"
}
},
{
$lookup: {
from: "users",
localField: "comments.sender_id",
foreignField: "_id",
as: "commented_user"
}
},
])
I am getting this result but what I want merge the commented_user into comments mean who has comments fetch the record.
[
{
"_id": ObjectId("5eeb02881982961ada625c7d"),
"commented_user": [
{
"_id": ObjectId("5e4d0973babf2b74ca868f4d"),
"first_name": "James",
"last_name": "Smith",
"timestamp": 1.582106995137e+12
}
],
"comments": [
{
"_id": ObjectId("5eeb08e26fb7f270e4077617"),
"date": 1.592461538923e+12,
"post_id": ObjectId("5eeb02881982961ada625c7d"),
"sender_id": ObjectId("5e4d0973babf2b74ca868f4d"),
"text": "Nice One "
}
],
"created_by": ObjectId("5e4e74eb380054797d9db623"),
"created_users": [],
"date": 1.589441206774e+12,
"title": "Covid19"
}
]
Would better if I get each commented detail under comments like below ?
"comments": [
{
"_id": ObjectId("5eeb08e26fb7f270e4077617"),
"date": 1.592461538923e+12,
"post_id": ObjectId("5eeb02881982961ada625c7d"),
"sender_id": ObjectId("5e4d0973babf2b74ca868f4d"),
"text": "Nice One ",
"commented_user": [
{
"_id": ObjectId("5e4d0973babf2b74ca868f4d"),
"first_name": "James",
"last_name": "Smith",
"timestamp": 1.582106995137e+12
}
],
}
],
Here is mongoplayground https://mongoplayground.net/p/uw0kMTChFa0

Answer 1

You could use $set and $unset or $project to format the outputs
Example with unset or Example with project
Updated Example. You need to unwind the comments array then perform lookup for each comment to get the commented user.
db.post.aggregate([
{
$lookup: {
from: "users",
localField: "created_by",
foreignField: "_id",
as: "created_users"
}
},
{
$lookup: {
from: "comments",
let: {
p_id: "$_id"
},
pipeline: [
{
$match: {
$expr: {
$eq: [
"$post_id",
"$$p_id"
]
}
}
}
],
as: "comments"
}
},
{
$unwind: "$comments"
},
{
$lookup: {
from: "users",
localField: "comments.sender_id",
foreignField: "_id",
as: "commented_user"
}
},
{
$set: {
"comments.commented_user": "$commented_user",
}
},
{
$unset: "commented_user"
}
])