- Tutorials
- Aggregation
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';
// Create a new MongoClient
const client = new MongoClient(url);
// Use connect method to connect to the Server
client.connect(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';
// Create a new MongoClient
const client = new MongoClient(url);
// Use connect method to connect to the Server
client.connect(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:
- an array of fields to group by
- a document with conditions for filterings
- an initial results document
- 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';
// Create a new MongoClient
const client = new MongoClient(url);
// Use connect method to connect to the Server
client.connect(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';
// Create a new MongoClient
const client = new MongoClient(url);
// Use connect method to connect to the Server
client.connect(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);
});
}