Aggregation operations in MongoDB allow you to process data records and return computed results. These operations group values from multiple documents, and perform a variety of operations on the grouped data to return a single result. MongoDB’s aggregation framework is powerful and flexible, enabling data analysis, transformation, and complex queries.

In this article, we’ll explore how to perform aggregation operations in MongoDB using Node.js. We’ll cover various aggregation approaches, provide step-by-step instructions to set up the application and illustrate with examples.

In MongoDB, the primary way to perform aggregation is through the aggregate method. This method can be used with various stages like $match, $group, $sort, $project, and many others to process the documents in the collection.

Common Aggregation Stages in MongoDB

• $match: Filters the documents to pass only those that match the specified condition(s).
• $group: Groups input documents by a specified identifier expression and apply the accumulator expressions.
• $sort: Sorts the documents.
• $project: Reshapes each document in the stream, such as by adding, removing, or renaming fields.

1. $match

The $match stage filters documents to pass only those that match the specified condition(s).

Syntax:

db.collection.aggregate([  { $match: { <condition> } }])

Example:

db.users.aggregate([
  { $match: { age: { $gte: 21 } } }
])

2. $group

The $group stage groups input documents by a specified identifier expression and applies the accumulator expressions.

Syntax:

db.collection.aggregate([  { $group: { _id: <expression>, <field1>: { <accumulator1>: <expression1> }, ... } }])

Example:

db.orders.aggregate([
  { $group: { _id: "$customerId", totalAmount: { $sum: "$amount" } } }
])

3. $sort

The $sort stage sorts all input documents and returns them in sorted order.

Syntax:

db.collection.aggregate([  { $sort: { <field1>: <sortOrder>, ... } }])

Example:

db.products.aggregate([
  { $sort: { price: -1 } }
])

4. $project

The $project stage reshapes each document in the stream, such as by adding, removing, or renaming fields.

Syntax:

db.collection.aggregate([  { $project: { <field1>: <expression1>, ... } }])

Example: To show

db.users.aggregate([
  { $project: { name: 1, age: 1, _id: 0 } }
])

Steps to Create Application

To create a Node.js application that uses MongoDB’s aggregation framework, follow these steps:

Step 1. Initialize Node.js Application

First, create a new directory for your application and initialize a new Node.js project.

mkdir mongo-aggregation
cd mongo-aggregation
npm init -y

Step 2. Install Required Modules

Install the mongodb package, which is the official MongoDB driver for Node.js.

npm install mongodb

Updated Dependencies:

"dependencies": {
    "mongodb": "^6.6.2",
  }

Example: To demonstrate creating a Node.js application that uses MongoDB’s aggregation framework.

// insertData.js
const { MongoClient } = require("mongodb");

async function insertSampleData() {
  const uri = "mongodb://localhost:27017";
  const client = new MongoClient(uri);

  try {
    await client.connect();

    const database = client.db("testdb");
    const collection = database.collection("users");

    const sampleData = [
      { name: "Alice", age: 25, city: "New York" },
      { name: "Bob", age: 30, city: "San Francisco" },
      { name: "Charlie", age: 25, city: "New York" },
      { name: "David", age: 30, city: "San Francisco" },
      { name: "Edward", age: 35, city: "Los Angeles" },
    ];

    await collection.insertMany(sampleData);
    console.log("Sample data inserted successfully");
  } finally {
    await client.close();
  }
}

insertSampleData().catch(console.error);

// index.js
const { MongoClient } = require("mongodb");

async function main() {
  const uri = "mongodb://localhost:27017";
  const client = new MongoClient(uri);

  try {
    await client.connect();

    const database = client.db("testdb");
    const collection = database.collection("users");

    // Example Aggregation: Group users by age and 
    // count the number of users in each age group
    const aggregationPipeline = [
      { $group: { _id: "$age", count: { $sum: 1 } } },
    ];

    const results = await collection.aggregate(aggregationPipeline).toArray();
    console.log("Aggregation results:", results);
  } finally {
    await client.close();
  }
}

main().catch(console.error);

Output:

Output