How to use $lookup on embedded document field in mogodb - node.js

Question

Please have a look, your help will be appriciated
var user = new Schema({
name: String,
});
var Comments = new Schema({
title : String
, body : String
,user_id : {type: Schema.Types.ObjectId, ref: 'user' }
, date : Date
});
var blog = new Schema({
author : String
, title : String
, body : String
, date : Date
, user_id :{type: Schema.Types.ObjectId, ref: 'user' }
, comments : [Comments]
});
db.blogs.aggregate([
{ $match : { "_id" : ObjectId("57e3b7f4409d80a508d52769") } },
{ $lookup: {from: "users", localField: "user_id", foreignField: "_id", as: "User"} },
])
this returns
[
{
"_id": "57e3b7f4409d80a508d52769",
"author": "Tariq",
"title": "MyfirstPost",
"body": "This is my first post",
"user_id": "57e3b763f7bc810c08f9467a",
"comments": [
{
"title": "hi",
"body": "again i am commenting on this",
"user_id": "57e3b763f7bc810c08f9467a",
"_id": "57e3c153409d80a508d5276b"
},
{
"title": "hi",
"body": "this is seond comment",
"user_id": "57e3b763f7bc810c08f9467a",
"_id": "57e3c8632ebca0ee0afb2ac6"
}
],
"__v": 0,
"User": [
{
"_id": "57e3b763f7bc810c08f9467a",
"name": "Tariq",
"username": "teekay",
"password": "123456",
"__v": 0
}
]
}
]
this return result by comparing blog table is and user table _id which is fine .. but I want to get userdetail with each comment by using user_id of “comments.user_id” blog collection and “_id” of collection
should be something like this
"_id": "57e3b7f4409d80a508d52769",
"author": "Tariq",
"title": "MyfirstPost",
"body": "This is my first post",
"user_id": "57e3b763f7bc810c08f9467a",
"comments": [
{
"title": "hi",
"body": "again i am commenting on this",
"user_id": "57e3b763f7bc810c08f9467a",
"_id": "57e3c153409d80a508d5276b",
"User": [
{
"_id": "57e3b763f7bc810c08f9467a",
"name": "Tariq",
"username": "teekay",
"password": "123456",
"__v": 0
}
]
},

Answer 1

You can run an aggregation operation of the pipeline:
db.blogs.aggregate([
{ "$unwind": "$comments" },
{
"$lookup": {
"from": "users",
"localField": "comments.user_id",
"foreignField": "_id",
"as": "comments.user"
}
},
{ "$unwind": "$comments.user" },
{
"$group": {
"_id": "$_id",
"author": { "$first": "$author" },
"title": { "$first": "$title" },
"body": { "$first": "$body" },
"comments": { "$push": "$comments" },
"user_id": { "$first": "$user_id" }
}
},
{
"$lookup": {
"from": "users",
"localField": "user_id",
"foreignField": "_id",
"as": "user"
}
},
{ "$unwind": "$user" },
])