Joining collections in MongoDB is done using the $lookup aggregation stage — think of it like a SQL JOIN. You can use it in Node.js with either:

1. ✅ Native MongoDB Driver

2. 🪄 Mongoose (with populate)

Let’s see both 👇

✅ 1. Using Native MongoDB Driver with $lookup

🧱 Sample Collections

users

json
{ _id: 1, name: "Alice", city_id: 101 }

cities

json
{ _id: 101, name: "New York" }

📦 Step 1: Install MongoDB

bash
npm install mongodb

🔗 Step 2: Use $lookup to Join

js
const { MongoClient } = require('mongodb');const uri = 'mongodb://localhost:27017';const client = new MongoClient(uri);async function joinCollections() {  try {    await client.connect();    const db = client.db('myDatabase');    const result = await db.collection('users').aggregate([      {        $lookup: {          from: 'cities',          // Collection to join          localField: 'city_id',   // Field in users          foreignField: '_id',     // Field in cities          as: 'city_info'          // Output array field        }      }    ]).toArray();    console.log(JSON.stringify(result, null, 2));  } finally {    await client.close();  }}joinCollections();

🪄 2. Using Mongoose with populate()

Mongoose makes joins easy using ref + populate.

📦 Step 1: Install Mongoose

bash
npm install mongoose

🔗 Step 2: Define Schemas and Join

js
const mongoose = require('mongoose');mongoose.connect('mongodb://localhost:27017/myDatabase')  .then(() => console.log('Connected!'));// Define City schemaconst citySchema = new mongoose.Schema({ name: String });const City = mongoose.model('City', citySchema);// Define User schema with a referenceconst userSchema = new mongoose.Schema({  name: String,  city: { type: mongoose.Schema.Types.ObjectId, ref: 'City' }});const User = mongoose.model('User', userSchema);// Join using populateasync function showJoinedData() {  const users = await User.find().populate('city');  console.log(users);  mongoose.disconnect();}showJoinedData();

✅ Summary