Mongodb group with Aggregation

var Mongo = require('mongodb');

var fs = require('fs'),
async = require('async');

Mongo.connect("mongodb://localhost:27017/aggregation_test?w=1", function(err, db) {
	var collection = db.collection('tree');
	var time = Date.now();
	collection.aggregate(
	{
		$project : {
			genre: 1
		}
	}, {
		$group : {
			_id: '$genre',
			totalByGenre: { $sum: 1}
		}
	},{
		$sort: {totalByGenre: -1}
	}, function(err, result) {
		console.log('Time Elapse %d ms', Date.now() - time); // env 350ms
		console.log(result);
		db.close();
	});
});

Mongodb group with Group

var Mongo = require('mongodb');

var fs = require('fs'),
async = require('async');

Mongo.connect("mongodb://localhost:27017/aggregation_test?w=1", function(err, db) {
	var collection = db.collection('tree');
	var time = Date.now();
	collection.group(
		{
			genre: true
		},
		{},
		{
			totalByGenre: 0
		},
		function ( curr, result ) {
			result.totalByGenre += 1;
		},
		function(err, result) {
			console.log('Time Elapse %d ms', Date.now() - time); // env 3500ms
			console.log(result);
			db.close();
		});
});