how to push data into array mongodb - mongodb

Question

Lookup with three collections re-manage data. I got confuse which to use. Please guide on this
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"
}
},
{
$lookup: {
from: "users",
localField: "comments.sender_id",
foreignField: "_id",
as: "commented_user"
}
}
])
There are three collection posts, user, and comments we would like to get who has comment for a post, Commented user should come under comments like this
[
{
"_id": ObjectId("5eeb02881982961ada625c7d"),
"commented_user": [
{
"_id": ObjectId("5e4d0973babf2b74ca868f4d"),
"first_name": "James",
"last_name": "Smith",
"timestamp": 1.582106995137e+12
},
{
"_id": ObjectId("5e4d0973babf2b74ca868f6d"),
"first_name": "Alex",
"last_name": "Jimy",
"timestamp": 1.582106995139e+12
}
],
"comments": [
{
"_id": ObjectId("5eeb08e26fb7f270e4077617"),
"date": 1.592461538924e+12,
"post_id": ObjectId("5eeb02881982961ada625c7d"),
"sender_id": ObjectId("5e4d0973babf2b74ca868f4d"),
"text": "Nice ",
"commented_user": {
"_id": ObjectId("5e4d0973babf2b74ca868f4d"),
"first_name": "James",
"last_name": "Smith",
"timestamp": 1.582106995137e+12
},
},
{
"_id": ObjectId("5eeb08e26fb7f270e4077618"),
"date": 1.592461538923e+12,
"post_id": ObjectId("5eeb02881982961ada625c7d"),
"sender_id": ObjectId("5e4d0973babf2b74ca868f6d"),
"text": "Nice One",
"commented_user": {
"_id": ObjectId("5e4d0973babf2b74ca868f6d"),
"first_name": "Alex",
"last_name": "Jimy",
"timestamp": 1.582106995137e+12
},
}
],
"created_by": ObjectId("5e4e74eb380054797d9db623"),
"created_users": [],
"date": 1.589441206774e+12,
"title": "Covid19"
}
]
here is my attempt https://mongoplayground.net/p/UwRjj-er0K5
Please help on this thanks

Answer 1

You can do something like this:
The strategy is $unwinding the results so we can match per comment and then reconstruct the former structure.
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"
}
},
{
$unwind: "$commented_user"
},
{
$addFields: {
comments: {
$mergeObjects: [
"$comments",
{
commented_user: "$commented_user"
}
]
}
}
},
{
$group: {
_id: "$_id",
comments: {
$push: "$comments"
},
created_by: {
$first: "$created_by"
},
created_users: {
$first: "$created_users"
},
date: {
$first: "$date"
},
title: {
$first: "$title"
},
}
}
])
Mongo Playground

Answer 2

Starting with your the code you have attempted, you can just move your second $lookup into the first $lookup's pipeline, then $unwind it
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", "$$p_id"] }
}
},
{ // second level $lookup
$lookup: {
from: "users",
localField: "sender_id",
foreignField: "_id",
as: "commented_user"
}
},
{ // $unwind to get single object instead of array
$unwind: "$commented_user"
}
],
as: "comments"
}
},
])
Mongo Playground
Note for OP: There is discussion to get the latest comment instead. With this approach you can $sort and $limit in the outermost $lookup
db.post.aggregate([
{
$lookup: {
from: "users",
localField: "created_by",
foreignField: "_id",
as: "created_users"
}
},
{
$lookup: {
from: "comments",
let: { p_id: "$_id" },
pipeline: [
{
$match: {
$expr: { $eq: ["$post_id", "$$p_id"] }
}
},
{
$sort: { date: -1 } // or { _id: -1 }
},
{
$limit: 1 // limit to only one element
},
{
$lookup: {
from: "users",
localField: "sender_id",
foreignField: "_id",
as: "commented_user"
}
},
{
$unwind: "$commented_user"
}
],
as: "comments"
}
},
{ // // $unwind to get single object instead of array
$unwind: "$comments"
}
])
Mongo Playground