Promise and nodejs MongoDB driver

Question

I would like to make a promise with the MongoDB driver. I wrote the following code:

var TaskBroker = function () {
  this.queueName = 'task_queue';
  this.rabbit = {};
  this.mongo = {};
};

TaskBroker.prototype.connectRabbit = function() {
  var self = this;

  return amqp.connect('amqp://localhost')
    .then(function(connection) {
      self.rabbit.connection = connection;
      return connection.createChannel()
    })
    .then(function(channel) {
      self.rabbit.channel = channel;
      return channel.assertQueue(self.queueName, {durable: true});
    })
};

TaskBroker.prototype.connectMongo = function() {
  console.log('Connect Mongo');
  var self = this;
  var defer = q.defer();
  MongoClient.connect('mongodb://127.0.0.1:27017/test', {}, defer.makeNodeResolver());
  return  defer.promise.then(function(db) {
    self.mongo.db = db;
    console.log('hello');
    return 42;
  });
};

TaskBroker.prototype.connect = function () {
  var self = this;
  return this.connectRabbit()
    .then(self.connectMongo);
};

Do you have any idea why I don't have the output hello when I call the method connect:

taskBroker.connect()
  .then(function(result) {
    console.log('Disconnected');
    taskBroker.disconnect();
});

Answer 1

Manually promisifying an API is dangerous, I suggest something along the lines of:

TaskBroker.prototype._connectMongo = Q.nfcall(MongoClient.connect,
                                             'mongodb://127.0.0.1:27017/test',
                                            {});
TaskBroker.prototype.connectMongo = function(){
   return this._connectMongo().then(function(db){
       console.log("Hello");
       // self.stuff...
       return 42;
   }).catch(function(e){
       console.err("connection error",e); // log the connection error, or handler err
       throw e; // don't mark as handled, propagate the error.
   });
};

With Bluebird promises, that'd look something like:

var MongoClient = Promise.promisifyAll(require("mongodb").MongoClient);

TaskBroker.prototype.connectMongo = function(){
    return MongoClient.connectAsync().then(... 
        // Bluebird will automatically track unhandled errors        
};

Answer 2

v2.0.36 of the node mongodb driver introduced first-class support for promises.

Here's an example from the official docs:

// A simple query showing skip and limit using a Promise.

var MongoClient = require('mongodb').MongoClient,
  test = require('assert');
MongoClient.connect('mongodb://localhost:27017/test', function(err, db) {

  // Create a collection we want to drop later
  var collection = db.collection('simple_limit_skip_query_with_promise');
  // Insert a bunch of documents for the testing
  collection.insertMany([{a:1, b:1}, {a:2, b:2}, {a:3, b:3}], {w:1}).then(function(result) {

    // Peform a simple find and return all the documents
    collection.find({})
      .skip(1).limit(1).project({b:1}).toArray().then(function(docs) {
        test.equal(1, docs.length);
        test.equal(null, docs[0].a);
        test.equal(2, docs[0].b);

        db.close();
    });
  });
});

By default, the es6-promise library is used, but you can override this when creating your DB connection:

var MongoClient = require('mongodb').MongoClient;

MongoClient.connect('mongodb://localhost:27017/test', {
  promiseLibrary: require('bluebird')
}, function(err, db) {
  // ...