a promise example

myController.js

const mongoose = require('mongoose');
const Store = mongoose.model('Store');
const StoreB = mongoose.model('StoreB');

// Tells package to use es6 promises
mongoose.Promise = global.Promise;

exports.createStore = (req, res, next) => {
  const store = new Store(req.body);
  const itemB = StoreB
    .findOne({ id: req.params._id})
    // Tighter Express Pattern:
    .catch(next);
    // Standard re-throw, technically not needed here*
    // .catch(err => {
    //   throw Error(err); // Needs `new`, or no stack trace is created, 
    // });
  
  // This line wont work, record_we_want_to_associate is a promise, not a StoreB
  // store.storeb_id = record_we_want_to_associate._id
  
  // For a dependent task, we access `record_we_want_to_associate`/itemB with `.then()`:
  itemB
    .then(record => {
      store.storeb_id = record._id;

      return store.save()
        // after save return all records
        .then(stores => Store.find())
        // then display them on index page    
        .then(stores => res.render('index', {stores: stores}))
    })
    // deal with errors
    .catch(next);
}