Node.js MongoDB Join

In the world of Node.js development, the ability to join collections in MongoDB signifies a leap towards creating more dynamic and interconnected data relationships. This functionality is essential for developers looking to build complex applications with interrelated datasets. In this section, we will delve into how to join collections in MongoDB using Node.js, with a focus on joining orders and products collections as a practical example.

Understanding MongoDB Joins with Node.js

MongoDB, a NoSQL database, is renowned for its flexibility and scalability, often using documents and collections instead of the traditional tables and rows found in relational databases. However, real-world applications frequently require the interrelation of data stored in separate collections. MongoDB addresses this need with the $lookup aggregation pipeline stage, which allows for a SQL-like join operation.

Preparing Our Collections: orders and products

Before we dive into the example, let’s set the stage with our two main actors: the orders and products collections.

  • The orders collection contains documents that represent orders made by customers, each order potentially involving multiple products.
  • The products collection houses documents that represent the products available for purchase, detailing their name, price, and other attributes.

Joining Collections in MongoDB using Node.js

To join these collections, we utilize the MongoDB Aggregation Framework, specifically the $lookup stage. This operation allows documents from one collection to be aggregated with documents from another collection.

Setting Up the Example

First, ensure you have MongoDB and Node.js installed and properly configured on your system. You will also need to have a MongoDB driver for Node.js installed in your project, which can be done using npm:

npm install mongodb

Next, let’s outline the documents within our orders and products collections for clarity:

  • orders collection:
  { "_id": 1, "product_id": 101, "quantity": 2 },
  { "_id": 2, "product_id": 102, "quantity": 1 },
  { "_id": 3, "product_id": 103, "quantity": 4 }
  • products collection:
  { "_id": 101, "name": "Laptop", "price": 900 },
  { "_id": 102, "name": "Smartphone", "price": 500 },
  { "_id": 103, "name": "Tablet", "price": 300 }

Example: Joining orders with products

Now, to perform a join operation between these collections, we’ll write a Node.js script that uses the MongoDB driver’s aggregation framework.

const { MongoClient } = require('mongodb');

async function joinOrdersWithProducts() {
  const uri = 'your_mongodb_connection_string';
  const client = new MongoClient(uri);

  try {
    await client.connect();
    const database = client.db('your_database_name');
    const orders = database.collection('orders');

    const aggregation = [
        $lookup: {
          from: 'products', // collection to join
          localField: 'product_id', // field from the orders collection
          foreignField: '_id', // field from the products collection
          as: 'product_details' // array field added to each order

    const joinedData = await orders.aggregate(aggregation).toArray();
  } finally {
    await client.close();


In this script, we connect to a MongoDB database and then perform an aggregation on the orders collection. The $lookup stage effectively joins each order with its corresponding product details from the products collection, adding a new array field product_details to each order document in the output.


Joining collections in MongoDB using Node.js provides a powerful mechanism to relate disparate data, offering insights and possibilities that isolated collections cannot. By mastering the $lookup aggregation pipeline stage, developers can construct more sophisticated data models and queries, propelling their applications to new heights of functionality and complexity. Through examples like the one above, it becomes evident how such techniques are indispensable in the toolkit of modern web developers.