using promises with bluebird

promises.js

var Promise = require('bluebird');

Promise.promisifyAll(City); // creates copies of every function with "Async" suffix... each return a promise
Promise.promisifyAll(Category);

Promise.all([
  City.getAsync(0),
  Category.getAsync(0) 
]).then(function (responses) {
  var cityResponse = responses[0];
  var categoryResponse = responses[1];
  //do stuff with your city and category... aka res.render
}, function (errors) {
  var cityGetError = errors[0];
  var categoryGetError = errors[1];
  //handle errors... aka res.status(500).send({error: 'something is broken!'})
});