Mongodb mongoose query get count of documents by reference - mongodb

Question

I have 2 collections in mongodb. articles and tags.
In articles, there can be multiple tags.
Following is the article schema:
const mongoose = require('mongoose');
const articleSchema = new mongoose.Schema({
title: {
type: String,
required: true,
trim: true
},
tags: [{
type: mongoose.Schema.Types.ObjectId,
required: true,
ref: 'Tag'
}]
}, {
timestamps: true
});
const Article = mongoose.model('Article', articleSchema);
module.exports = Article;
Following is the tag schema:
const mongoose = require('mongoose');
const tagSchema = new mongoose.Schema({
title: {
type: String,
required: true,
trim: true,
unique: true
}
}, {
timestamps: true
});
const Tag = mongoose.model('Tag', tagSchema);
module.exports = Tag;
From these collections I wanted to show a simple column chart which shows
how many articles are there against a tag.
I am trying to get data in format like this:
const data = [
{ title: 'Javascript', count: 20 },
{ title: 'ReactJs', count: 12 },
{ title: 'NodeJs', count: 5 }
];
I have tried aggregate, $lookup but not able to find solution.
Also tried this answer
Following I have tried but its not giving desired output.
const result = await Tag.aggregate([
{
$lookup:
{
from: "articles",
localField: "_id",
foreignField: "tags",
as: "articles"
}
}
])
It gives output like this, it returns articles array against tag but I need count of articles only.
[{
"_id": "5f6f39c64250352ec80b0e10",
"title": "ReactJS",
articles: [{ ... }, { ... }]
},{
"_id": "5f6f40325716952d08a6813c",
"title": "Javascript",
articles: [{ ... }, { ... },{ ... }, { ... }]
}]
If anyone knows solution, please let me know. Thank you.

Answer 1

$lookup with articles collection
$project to show required fields and get total size of articles array using $size
const result = await Tag.aggregate([
{
$lookup: {
from: "articles",
localField: "_id",
foreignField: "tags",
as: "articles"
}
},
{
$project: {
_id: 0,
title: 1,
count: { $size: "$articles" }
}
}
])
Playground