Aggregation

Overview

Aggregation operations process data records and return computed results. Aggregation operations group values from multiple documents together, and can perform a variety of operations on the grouped data to return a single result.

The Aggregation Pipeline

The aggregation pipeline is a framework for data aggregation modeled on the concept of data processing pipelines. Documents enter a multi-stage pipeline that transforms the documents into aggregated results.

For a full explanation and a complete list of pipeline stages and operators, see the manual:

The following example uses the aggregation pipeline on the restaurant sample dataset to find a list of restaurants located in the Bronx, grouped by restaurant category.

const MongoClient = require('mongodb').MongoClient;
const assert = require('assert');

// Connection URL
const url = 'mongodb://localhost:27017';

// Database Name
const dbName = 'myproject';

// Use connect method to connect to the Server
MongoClient.connect(url, function(err, client) {
  assert.equal(null, err);
  console.log("Connected correctly to server");

  const db = client.db(dbName);

  simplePipeline(db, function() {
    client.close();
  });
});

function simplePipeline(db, callback) {
  const collection = db.collection( 'restaurants' );
  collection.aggregate(
      [ { '$match': { "borough": "Bronx" } },
        { '$unwind': '$categories'},
        { '$group': { '_id': "$categories", 'Bronx restaurants': { '$sum': 1 } } }
      ],
    function(err, cursor) {
        assert.equal(err, null);

        cursor.toArray(function(err, documents) {
          console.log(documents)
          callback(documents);
        });
      }
  );
}

Inside the aggregate method, the first pipeline stage filters out all documents except those with 5 in the stars field. The second stage unwinds the categories field, which is an array, and treats each item in the array as a separate document. The third stage groups the documents by category and adds up the number of matching 5-star results.

Single Purpose Aggregation Operations

MongoDB provides helper methods for some aggregation functions, including count, group, and distinct.

Count

The following example demonstrates how to use the count method to find the total number of documents which have the exact array [ 'Chinese', 'Seafood' ] in the categories field.

const MongoClient = require('mongodb').MongoClient;
const assert = require('assert');

// Connection URL
const url = 'mongodb://localhost:27017';

// Database Name
const dbName = 'myproject';

// Use connect method to connect to the Server
MongoClient.connect(url, function(err, client) {
  assert.equal(null, err);
  console.log("Connected correctly to server");

  const db = client.db(dbName);

  simpleCount(db, function() {
    client.close();
  });
});

function simpleCount(db, callback) {
  const collection = db.collection( 'restaurants' );
  collection.count({ 'categories': [ 'Chinese', 'Seafood' ] },	  
	  function(err, result) {
        assert.equal(err, null);
        console.log(result)
        callback(result);
      }
  );
}

Group

The following example uses the group method with four arguments:

1. an array of fields to group by
2. a document with conditions for filterings
3. an initial results document
4. a reduce function

The example groups the results by number of stars where the categories array is ['Peruvian'].

const MongoClient = require('mongodb').MongoClient;
const assert = require('assert');

// Connection URL
const url = 'mongodb://localhost:27017';

// Database Name
const dbName = 'myproject';

// Use connect method to connect to the Server
MongoClient.connect(url, function(err, client) {
  assert.equal(null, err);
  console.log("Connected correctly to server");

  const db = client.db(dbName);

  simpleGroup(db, function() {
    client.close();
  });
});

function simpleGroup(db, callback) {
    const collection = db.collection( 'restaurants' );
    collection.group( ['stars'], 
                      { 'categories': ['Peruvian'] }, 
                      { 'total': 0 },
                      "function ( curr, result ) { result.total++ }", 
      function(err, result) {
        assert.equal(err, null);
        console.log(result)
        callback(result);
      }
  );
}

Distinct

The distinct helper method eliminates results which contain values and returns one record for each unique value.

The following example returns a list of unique values for the categories field in the restaurants collection:

const MongoClient = require('mongodb').MongoClient;
const assert = require('assert');

// Connection URL
const url = 'mongodb://localhost:27017';

// Database Name
const dbName = 'myproject';

// Use connect method to connect to the Server
MongoClient.connect(url, function(err, client) {
  assert.equal(null, err);
  console.log("Connected correctly to server");

  const db = client.db(dbName);

  simpleDistinct(db, function() {
    client.close();
  });
});

function simpleDistinct(db, callback) {
  const collection = db.collection( 'restaurants' );
  collection.distinct( 'categories', function(err, result) {
    assert.equal(err, null);
    console.log(result)
    callback(result);
  });
}