Запросы зависают при использовании mongoose.createConnection() vs mongoose.connect()

Работает:

var mongoose = require('mongoose');

var db = function() {
  return {
    config: function(conf) {
      mongoose.connect('mongodb://' + conf.host + '/' + conf.database);
      var db = mongoose.connection;
      db.on('error', console.error.bind(console, 'connection error:'));
      db.once('open', function callback() {
        console.log('db connection open');
      });
    }
  };
};

module.exports = db();

Не работает:

var mongoose = require('mongoose');

var db = function() {
  return {
    config: function(conf) {
      var db = mongoose.createConnection('mongodb://' + conf.host + '/' + conf.database);
      db.on('error', console.error.bind(console, 'connection error:'));
      db.once('open', function callback() {
        console.log('db connection open');
      });
    }
  };
};

module.exports = db();

Код вставки:

'use strict';


var mongoose = require('mongoose'),
User = mongoose.model('User'),
p = require('../lib/promise');

...
app.post('/user', function (req, res) {
  res.format({
    json: function () {
      //extract the user from the req
      try {
        var user = new User();
        user.firstName = req.body.firstName;
        user.lastName = req.body.lastName;
        user.userName = req.body.userName;
        user.password = req.body.password;
        user.email = req.body.email;

        user.save(function(err, data) {
        //omitted
...