Populate field in an array of objects MongoDB and Mongoose - node.js

Question

I'm building a comment system for my website. I've created this schema for the article and it's comments:
let articleSchema = mongoose.Schema({
language: {
type: String,
default: "english"
},
slug: {
type: String,
required: true
},
image: {
type: String,
required: true
},
title: {
type: String,
required: true
},
tags: {
type: [String],
required: true
},
author: {
type: mongoose.Schema.Types.ObjectId,
ref: "User",
required: true
},
date: {
type: Date,
default: Date.now()
},
description: {
type: String,
required: false
},
body: {
type: String,
required: true
},
translation: [
{
language: {
type: String,
required: true
},
title: {
type: String,
required: true
},
tags: {
type: [String],
required: true
},
description: {
type: String,
required: false
},
body: {
type: String,
required: true
}
}
],
comments: [{
author: {
type: mongoose.Schema.Types.ObjectId,
ref: "User"
},
date: {
type: Date,
default: Date.now()
},
body: {
type: String,
required: true
}
}]
});
Now what I want to do is to populate every comment author field.
Let's say this is the article with a comment:
{
"comments": [
{
"author": "5f71d3b575f9f800943903af",
"date": "some date here",
"body": "comment body"
}
]
}
Now what I want to get after I populated all of this is this:
{
"comments": [
{
"author": {
"username": "Username",
"avatar": "fox"
},
"date": "some date here",
"body": "comment body"
}
]
}
How can I do that?
Here's my current code:
router.get('/article/:id', async (req, res) => {
const { id: articleId } = req.params;
const lang = req.i18n.language;
const langName = new Intl.DisplayNames(['en'], { type: "language" }).of(lang).toLowerCase();
try {
console.log(langName);
if (req.i18n.language === "en") {
var article = await Article.findById(articleId)
.populate([
{path:'comments.author', select:'+photo +username'},
{path:'author', select:'+photo +username'}
])
.select("title image tags author date description body comments");
} else {
var article = await Article
.aggregate([
{
$match: {
"_id": ObjectId(articleId)
}
}, {
$unwind: "$translation"
}, {
$match: {
"translation.language": langName
}
}, {
$group: {
_id: "$_id",
image: { $first: "$image" },
title: { $first: "$translation.title" },
tags: { $first: "$translation.tags" },
author: { $first: "$author" },
date: { $first: "$date" },
description: { $first: "$translation.description" },
body: { $first: "$translation.body" },
comments: { $first: "$comments" }
}
},
{
$lookup: {
from: "users",
localField: "author",
foreignField: "_id",
as: "author"
}
},
{
$unwind: {
path: "$author"
}
},
{ ///////////////////// Here I need to populate the comment author. How can I do it?
$lookup: {
from: "users",
localField: "comments.author",
foreignField: "_id",
as: "comments.author"
}
},
{
$unwind: {
path: "$comments.author"
}
},
{
$project: {
image: 1,
title: 1,
tags: 1,
date: 1,
description: 1,
body: 1,
// comments: 1,
"comments.date": 1,
"comments.body": 1,
"comments.author.username": 1,
"comments.author.avatar": 1,
"author.username": 1,
"author.avatar": 1
}
}
], function(err, result) {
console.log(err)
return result;
});
console.log(article[0].comments[0]);
console.log(article);
}
if (!article) throw new Error();
} catch {
return res.status(404).render('errors/404');
}
res.render('articles/article', {
article: article[0]
});
});

Answer 1

So after some digging I came up with this solution:
router.get('/article/:id', async (req, res) => {
const { id: articleId } = req.params;
const lang = req.i18n.language;
const langName = new Intl.DisplayNames(['en'], { type: "language" }).of(lang).toLowerCase();
try {
if (req.i18n.language === "en") {
var article = await Article.findById(articleId)
.populate([
{path:'comments.author', select:'+photo +username'},
{path:'author', select:'+photo +username'}
])
.select("title image tags author date description body comments");
} else {
var article = await Article
.aggregate([
{
$match: {
"_id": ObjectId(articleId)
}
}, {
$unwind: "$translation"
}, {
$match: {
"translation.language": langName
}
},
{
$lookup: {
from: "users",
localField: "author",
foreignField: "_id",
as: "author"
}
},
{
$unwind: {
path: "$author"
}
},
{
$unwind: {
path: "$comments"
}
},
{
$lookup: {
from: "users",
localField: "comments.author",
foreignField: "_id",
as: "comments.author"
}
},
{
$group: {
_id: "$_id",
root: { $mergeObjects: '$$ROOT' },
image: { $first: "$image" },
title: { $first: "$translation.title" },
tags: { $first: "$translation.tags" },
author: { $first: "$author" },
date: { $first: "$date" },
description: { $first: "$translation.description" },
body: { $first: "$translation.body" },
comments: { $push: "$comments" }
}
},
{
$project: {
image: 1,
title: 1,
tags: 1,
date: 1,
description: 1,
body: 1,
"comments.date": 1,
"comments.body": 1,
"comments.author.username": 1,
"comments.author.avatar": 1,
"author.username": 1,
"author.avatar": 1
}
}
]);
}
if (!article) throw new Error();
} catch {
return res.status(404).render('errors/404');
}
res.render('articles/article', {
article: article[0]
});
});