The MongoDB aggregation framework in Node.js performs data aggregation operations such as filtering, grouping and transforming documents within a collection. This is achieved using aggregation pipelines. An aggregation pipeline consists of multiple stages, with an output of using stage being passed to the next stage as input. This allows to processing of the data and returns computed results. An Aggregation pipeline can have one or more stages, each processing using expression operators.

Key Stages in Aggregation Pipeline

The MongoDB aggregation framework provides several stages that can be used to perform various data processing activities. Some of the key stages include:

1. $match

The `$match` stage filters documents based on a specified condition. It is similar to the find operation.

Syntax:

{ $match: { <field>: <value> } }

Example:

{ $match: { age: { $gte: 18 } } }

This will output only the documents where the age field is greater than or equal to 18.

2. $group

The $group stage groups documents by a specified identifier and applies accumulator expressions to each group.

Syntax:

{ $group: { _id: <expression>, <field>: { <accumulator>: <expression> } } }

Example:

{ $group: { _id: "$city", totalPopulation: { $sum: "$population" } } }

This will group documents by the city field and calculate the total population for each city.

3. $project

The `$project` stage reshapes each document by including, excluding, or adding new fields.

Syntax:

{ $project: { <field>: <1 or 0> } }

Example:

{ $project: { name: 1, age: 1, _id: 0 } }

This will include only the name and age fields in the output documents.

4. $sort

The `$sort` stage sorts input documents using a given field in either ascending (1) or descending (-1) order.

Syntax:

{ $sort: { <field>: 1 or -1 } }

Example:

{ $sort: { age: -1 } }

This will sort the documents by the age field in descending order.

5. $limit

The `$limit` stage limits the number of documents passed as output to the next stage.

Syntax:

{ $limit: <number> }

Example:

{ $limit: 5 }

This will output the first 5 documents to the next stage.

6. $skip

The $skip stage skips the first N documents and passes the remaining documents.

Syntax:

{ $skip: <number> }

Example

{ $skip: 10 }

This will skip the first 10 documents and output the rest.

Steps to Implement MongoDB Aggregation Framework

Step 1: Create a node.js project and Install Required Modules

To utilize the MongoDB aggregation framework in a Node.js application, Create a node.js project and install the mongodb package using:

npm init
npm install mongodb

Step 2: Create an index.js File and modify index.js file

Create a file named index.js and add the following code:

const { MongoClient } = require('mongodb');
const url = 'mongodb://localhost:27017';
const client = new MongoClient(url);
const dbName = 'mydatabase';
async function main() {
    await client.connect();
    console.log('Connected successfully to server');
    const db = client.db(dbName);
    const collection = db.collection('documents');

    const pipeline = [
        { $match: { age: { $gte: 18 } } },
        { $group: { _id: "$city", totalPopulation: { $sum: "$population" } } },
        { $sort: { totalPopulation: -1 } },
        { $limit: 5 }
    ];

    const result = await collection.aggregate(pipeline).toArray();
    console.log(result);
}
main();

Step 3: Update package.json

Modify the package.json to include all your required dependencies and define the start command.

  "scripts": {
      "start": "node index.js",
      "test": "echo \"Error: no test specified\" && exit 1"
  },
  "dependencies": {
      "mongodb": "^6.8.0"
  }

Step 4: Run the application

Insert some placeholder data into your MongoDB collection and run the application using ‘npm start’.

Example data

[
    {
        "name": "John",
        "age": 25,
        "city": "New York",
        "population": 10000
    },
    {
        "name": "Jane",
        "age": 30,
        "city": "New York",
        "population": 10000
    },
    {
        "name": "Alice",
        "age": 22,
        "city": "Los Angeles",
        "population": 9000
    },
    {
        "name": "Bob",
        "age": 19,
        "city": "Chicago",
        "population": 5000
    },
    {
        "name": "Charlie",
        "age": 35,
        "city": "San Francisco",
        "population": 7000
    }
]

Run the application using below command:

npm start

Output:

aggregation pipeline output

MongoDB Aggregation Framework – FAQs

What is the difference between the $match stage and the find method in MongoDB?

While both find method and $match stage are designed to filter data, $match stage is specifically designed to be used in pipelines to combine with other stages.

Can we use multiple stages in a single aggregation pipeline?

Yes, we can use multiple stages in a single aggregation pipeline. Each stage processes the documents and passes the output results to the next stage in the sequence.

How to use the MongoDB aggregation framework with other Node.js frameworks like Express.js?

We can perform aggregation operations and handle the results within our routes and middleware in Express.js, by integrating the mongodb package with the application.