Bluebird Promises with Mongoose for MongoDB

We are going to take common Mongoose use cases for finding and creating documents and going to use Bluebird (or more generally, any promise library that follows Promises/A+) to migrate from the typical callback method to promises.

Many sites are showing simple examples of promises but sometimes you just:

• need an example specific to what you are doing to understand it
• need to just get going in it without fully understanding it

I assume you have a basic understanding of Bluebird promises and an intermediate understanding of Mongoose otherwise these examples won’t make sense. I also assume you are convinced you need/want/should use promises instead of callbacks for Mongoose in Node. I am not here to convince you that you need to use promises.

Save a document

Create a new document (review) in the database

Callback Style

   Review
   .create(review, function(err, savedReview){
    if (!err && savedReview){
      // we created the savedReview successfully
      res.status(200).send(savedReview);
    } else {
      res.status(500).send(err);
    }
  });

Promise Style

  var Promise = require('bluebird');
  Promise.promisifyAll(Review);

  Review
  .createReviewAsync(review)
  .then(function(savedReview){
    res.status(200).send(savedReview);
  })
  .catch(function(err){
    res.status(500).send(err);
  });

Find document using mPromise (Mongoose Promise)

Find a user in the database

Callback style

User
.findOne({username:username})
.exec(function(err, user){
  if (err){
    res.status(500).send(err);
  } else {
    res.status(200).send(user);
  }
});

Promise style

  var Promise = require('bluebird');
  Promise.resolve(User.findOne({username:username}).exec())
  .then(function(user){
    res.status(200).send(user);
  })
  .then(undefined, function(err){
    res.status(500).send(err);
  });