Gist to update several items in series in a huge MongoDB collection (millions of records) with Node.js and Async module

node-async-mongodb-multiple-updates.js

'use strict';
/**
 * Module dependencies.
 */

var async = require('async'),
    mongoose = require('mongoose'),
    ObjectId = mongoose.Types.ObjectId;

var db = mongoose.connect('mongodb://localhost:27017/test', {}, function (err) {
    if (err) {
        console.error('\x1b[31m', 'Could not connect to MongoDB!');
        console.log(err);
    } else {
        console.log('Connected to MongoDB: ' + config.db);

        // This is a huge collection you have to update some fields
        var Hugecollection = mongoose.model('Hugecollection');

        // First of all count the total items
        Hugecollection.count({}, function (err, tot) {
            if (err) return console.log(err);

            var skip = 0,
                limit = 10000
                pages = (tot / limit).toFixed(0),
                a = [];

            console.log('Total items: ' + tot + ' - Total pages: ' + pages);

            // Create an array of the page numbers to be queried
            for (skip = 0; skip <= pages; skip++) {
                a.push(skip);
            }

            // Give the page numbers array to an eachSeries method from Async, 
            // It will loop through them asking a number of limited (10000) items per time
            // that's better then query for all items when you have a lot
            async.eachSeries(a, function (key, done) {
                // Find each batch of items
                Hugecollection.find({}).select({
                    _id: 1,
                    fieldtobechanged1: 1,
                    fieldtobechanged2: 1
                }).limit(limit).skip(key * limit).sort({
                    _id: 1
                }).exec(function (err, items) {
                    if (err) return console.log(err);

                    console.log("Doing: " + key + "/" + pages + " - TEST THIS: " + items[0]._id);

		    // Asynchrously loop each item in the batch and do your changes
                    async.each(items, function (item, callback) {
                        if (item.fieldtobechanged1 && item.fieldtobechanged2) {
                            var doYourChange = function (value) {
                            	return value;
                            },
                            fieldtobechanged1 = doYourChange(item.fieldtobechanged1);
                            fieldtobechanged2 = doYourChange(item.fieldtobechanged2);

			    // Now update the item with your changes and you are done
                            Hugecollection.findOneAndUpdate({
                                _id: new ObjectId(item._id)
                            }, {
                                $set: {
                                    fieldtobechanged1: fieldtobechanged1,
                                    fieldtobechanged2: fieldtobechanged2
                                }
                            }, function (err, updated) {
                                callback();
                            });
                        } else {
                            callback();
                        }
                    }, done()); // done is call when all items are updated!
                });
            }, function (err) {
                console.log('Done!');
                mongoose.disconnect();
            });
        });
    }
});