ok.js

// OUTPUT:

// Executing: DROP TABLE IF EXISTS `Persons`;
// Executing: CREATE TABLE IF NOT EXISTS `Persons` (`name` VARCHAR(255), `id` INTEGER NOT NULL auto_increment , `createdAt` DATETIME NOT NULL, `updatedAt` DATETIME NOT NULL, PRIMARY KEY (`id`)) ENGINE=InnoDB;
// Executing: DROP TABLE IF EXISTS `Pets`;
// Executing: CREATE TABLE IF NOT EXISTS `Pets` (`name` VARCHAR(255), `id` INTEGER NOT NULL auto_increment , `createdAt` DATETIME NOT NULL, `updatedAt` DATETIME NOT NULL, `PersonId` INTEGER, PRIMARY KEY (`id`)) ENGINE=InnoDB;
// Executing: INSERT INTO `Persons` (`name`,`id`,`createdAt`,`updatedAt`) VALUES ('Luke',NULL,'2013-05-29 11:00:37','2013-05-29 11:00:40');
// Executing: INSERT INTO `Pets` (`name`,`id`,`createdAt`,`updatedAt`,`PersonId`) VALUES ('Bob',NULL,'2013-05-29 11:00:37','2013-05-29 11:00:40',NULL);
// Executing: SELECT * FROM `Pets` WHERE `Pets`.`PersonId`=1;
// Executing: UPDATE `Pets` SET `name`='Bob',`id`=1,`createdAt`='2013-05-29 11:00:37',`updatedAt`='2013-05-29 11:00:40',`PersonId`=1 WHERE `id`=1
// Executing: SELECT * FROM `Pets` WHERE `Pets`.`PersonId`=1;
// my pets: Bob

var Sequelize = require(__dirname + "/../../index")
  , config    = require(__dirname + "/../../spec/config/config")
  , sequelize = new Sequelize(config.database, config.username, config.password, {logging: true})
  , Person    = sequelize.define('Person', { 
    name: Sequelize.STRING
  })
  , Pet       = sequelize.define('Pet',    { name: Sequelize.STRING })

Person.hasMany(Pet)

var chainer = new Sequelize.Utils.QueryChainer
  , person  = Person.build({ name: 'Luke' })
  , pet     = Pet.build({ name: 'Bob' })

sequelize.sync({force:true}).on('success', function() {
  chainer
    .add(person.save())
    .add(pet.save())

  chainer.run().on('success', function() {
    person.setPets([pet]).on('success', function() { person.getPets().on('success', function(pets) {
      console.log("my pets: " + pets.map(function(p) { return p.name }))
    })})
  }).on('failure', function(err) {
    console.log(err)
  })
})

wrong.js

// OUTPUT:

// Executing: DROP TABLE IF EXISTS `Persons`;
// Executing: CREATE TABLE IF NOT EXISTS `Persons` (`name` VARCHAR(255), `user_id` INTEGER auto_increment , `createdAt` DATETIME NOT NULL, `updatedAt` DATETIME NOT NULL, PRIMARY KEY (`user_id`)) ENGINE=InnoDB;
// Executing: DROP TABLE IF EXISTS `Pets`;
// Executing: CREATE TABLE IF NOT EXISTS `Pets` (`name` VARCHAR(255), `id` INTEGER NOT NULL auto_increment , `createdAt` DATETIME NOT NULL, `updatedAt` DATETIME NOT NULL, `PersonId` INTEGER, PRIMARY KEY (`id`)) ENGINE=InnoDB;
// Executing: INSERT INTO `Persons` (`name`,`user_id`,`createdAt`,`updatedAt`) VALUES ('Luke',NULL,'2013-05-29 10:57:58','2013-05-29 10:57:58');
// Executing: INSERT INTO `Pets` (`name`,`id`,`createdAt`,`updatedAt`,`PersonId`) VALUES ('Bob',NULL,'2013-05-29 10:57:58','2013-05-29 10:57:58',NULL);
// Executing: SELECT * FROM `Pets` WHERE `Pets`.`PersonId` IS NULL;
// Executing: SELECT * FROM `Pets` WHERE `Pets`.`PersonId` IS NULL;
// my pets: Bob


var Sequelize = require(__dirname + "/../../index")
  , config    = require(__dirname + "/../../spec/config/config")
  , sequelize = new Sequelize(config.database, config.username, config.password, {logging: true})
  , Person    = sequelize.define('Person', { 
    name: Sequelize.STRING,
    user_id: {
      type: Sequelize.INTEGER,
      primaryKey: true,
      autoIncrement: true
    }
  })
  , Pet       = sequelize.define('Pet',    { name: Sequelize.STRING })

Person.hasMany(Pet)

var chainer = new Sequelize.Utils.QueryChainer
  , person  = Person.build({ name: 'Luke' })
  , pet     = Pet.build({ name: 'Bob' })

sequelize.sync({force:true}).on('success', function() {
  chainer
    .add(person.save())
    .add(pet.save())

  chainer.run().on('success', function() {
    person.setPets([pet]).on('success', function() { person.getPets().on('success', function(pets) {
      console.log("my pets: " + pets.map(function(p) { return p.name }))
    })})
  }).on('failure', function(err) {
    console.log(err)
  })
})