With Sequelize you can also specify associations between multiple classes. Doing so will help you to easily access and set those associated objects. The library therefore provides for each defined class different methods, which are explained in the following chapters.

Note: Associations with models that use custom primaryKeys (so not the field 'id') are currently unsupported.

One-To-One associations

One-To-One associations are connecting one source with exactly one target. In order to define a proper database schema, Sequelize utilizes the methods belongsTo and hasOne. You can use them as follows:

var User = sequelize.define('User', {/* ... */})
var Project = sequelize.define('Project', {/* ... */})
// One-way associations
  In this example hasOne will add an attribute ProjectId to the User model!
  Furthermore, Project.prototype will gain the methods getUser and setUser according
  to the first parameter passed to define. If you have underscore style
  enabled, the added attribute will be project_id instead of ProjectId.
  You can also define the foreign key, e.g. if you already have an existing
  database and want to work on it:
Project.hasOne(User, { foreignKey: 'initiator_id' })
  Because Sequelize will use the model's name (first parameter of define) for
  the accessor methods, it is also possible to pass a special option to hasOne:
Project.hasOne(User, { as: 'Initiator' })
// Now you will get Project#getInitiator and Project#setInitiator
// Or let's define some self references
var Person = sequelize.define('Person', { /* ... */})
Person.hasOne(Person, {as: 'Father'})
// this will add the attribute FatherId to Person
// also possible:
Person.hasOne(Person, {as: 'Father', foreignKey: 'DadId'})
// this will add the attribute DadId to Person
// In both cases you will be able to do:
// If you need to join a table twice you can double join the same table
  .hasOne(Game, {foreignKey : 'homeTeamId'});
  .hasOne(Game, {foreignKey : 'awayTeamId'});
// Since v1.3.0 you can also chain associations:

To get the association working the other way around (so from User to Project), it's necessary to do this:

var User = sequelize.define('User', {/* ... */})
var Project = sequelize.define('Project', {/* ... */})
// One-way back associations
  In this example belongsTo will add an attribute UserId to the Project model!
  That's the only difference to hasMany. Self references are working the very same way!

One-To-Many associations

One-To-Many associations are connecting one source with multiple targets. The targets however are again connected to exactly one specific source.

var User = sequelize.define('User', {/* ... */})
var Project = sequelize.define('Project', {/* ... */})
// OK. Now things get more complicated (not really visible to the user :)).
// First let's define a hasMany association
Project.hasMany(User, {as: 'Workers'})

This will add the attribute ProjectId or project_id to User. Instances of Project will get the accessors getWorkers and setWorkers. We could just leave it the way it is and let it be a one-way association. But we want more! Let's define it the other way around by creating a many to many assocation in the next section:

Many-To-Many associations

Many-To-Many associations are used to connect sources with multiple targets. Furthermore the targets can also have connections to multiple sources.

// again the Project association to User
// now comes the association between User and Project

This will remove the attribute ProjectId (or project_id) from User and create a new model called ProjectsUsers with the equivalent foreign keys ProjectId(or project_id) and UserId (or user_id). Whether the attributes are camelcase or not depends on the two models joined by the table (in this case User and Project).

This will add methods getUsers, setUsers, addUsers to Project, and getProjects, setProjects and addProject to User.

Sometimes you may want to rename your models when using them in associations. Let's define users as workers and projects as tasks by using the alias (as) option:

User.hasMany(Project, { as: 'Tasks', through: 'worker_tasks' })
Project.hasMany(User, { as: 'Workers', through: 'worker_tasks' })

Notice how we used the through option together with the alias in the code above. This is needed to tell sequelize that the two hasMany calls are actually two sides of the same association. If you don't use an alias (as shown in the first example of this section) this matching happens automagically, but with aliassed assocations through is required.

Of course you can also define self references with hasMany:

Person.hasMany(Person, { as: 'Children' })
// This will create the table ChildrenPersons which stores the ids of the objects.
// You can also reference the same Model without creating a junction
// table (but only if each object will have just one 'parent'). If you need that,
// use the option foreignKey and set through to null
Comment.hasMany(Comment, { as: 'Children', foreignKey: 'ParentId', through: null })

By default, sequelize will handle everything related to the join table for you. However, sometimes you might want some more control over the table. This is where th ethrough options comes in handy.

If you just want to control the name of the join table, you can pass a string:

Project.hasMany(User, {through: 'project_has_users'})
User.hasMany(Project, {through: 'project_has_users'})

If you want additional attributes in your join table, you can define a model for the join table in sequelize, before you define the association, and then tell sequelize that it should use that model for joining, instead of creating a new one:

User = sequelize.define('User', {})
Project = sequelize.define('Project', {})
UserProjects = sequelize.define('UserProjects', {
    status: DataTypes.STRING
User.hasMany(Project, { through: UserProjects })
Project.hasMany(User, { through: UserProjects })

To add a new project to a user and set it's status, you pass an extra object to the setter, which contains the attributes for the join table

user.addProject(project, { status: 'started' })

By default the code above will add ProjectId and UserId to the UserProjects table, and_remove any previsouly defined primary key attribute _- the table will be uniquely identified by the combination of the keys of the two tables, and there is no reason to have other PK columns. To enforce a primary key on the UserProjects model you can add it manually.

UserProjects = sequelize.define('UserProjects', {
    id: {
      type: Sequelize.INTEGER,
      primaryKey: true,
      autoIncrement: true
    status: DataTypes.STRING

Naming strategy

By default sequelize will use the model name (the name passed to sequelize.define) to figure out the name of the model when used in associations. For example, a model named user will add the functions get/set/add User to instances of the associated model, and a property named .user in eager loading, while a model named User will add the same functions, but a property named .User (notice the upper case U) in eager loading.

As we've already seen, you can alias models in associations using as. In single assocations (has one and belongs to), the alias should be singular, while for many associations (has many) it should be plural. Sequelize then uses the inflection library to convert the alias to its singular form. However, this might not always work for irregular or non-english words. In this case, you can provide both the plural and the singular form of the alias:

User.hasMany(Project, { as: { singular: 'task', plural: 'tasks' }}) 
// Notice that inflection has no problem singularizing tasks, this is just for illustrative purposes.

If you know that a model will always use the same alias in associations, you can provide it when creating the model

var Project = sequelize.define('project', attributes, {
  name: {
    singular: 'task',
    plural: 'tasks',

This will add the functions add/set/get Tasks to user instances.

Associating objects

Because Sequelize is doing a lot of magic, you have to call Sequelize.sync after setting the associations! Doing so will allow you the following:

// save them... and then:
project.setTasks([task1, task2]).success(function() {
  // saved!
// ok now they are save... how do I get them later on?
project.getTasks().success(function(associatedTasks) {
  // associatedTasks is an array of tasks
// You can also pass filters to the getter method.
// They are equal to the options you can pass to a usual finder method.
project.getTasks({ where: 'id > 10' }).success(function(tasks) {
  // tasks with an id greater than 10 :)
// You can also only retrieve certain fields of a associated object.
// This example will retrieve the attibutes "title" and "id"
project.getTasks({attributes: ['title']}).success(function(tasks) {
  // tasks with an id greater than 10 :)

To remove created associations you can just call the set method without a specific id:

// remove the association with task1
project.setTasks([task2]).success(function(associatedTasks) {
  // you will get task2 only
// remove 'em all
project.setTasks([]).success(function(associatedTasks) {
  // you will get an empty array
// or remove 'em more directly
project.removeTask(task1).success(function() {
  // it's gone
// and add 'em again
project.addTask(task1).success(function() {
  // it's back again

You can of course also do it vice versa:

// project is associated with task1 and task2
task2.setProject(null).success(function() {
  // and it's gone

For hasOne/belongsTo its basically the same:

Task.hasOne(User, {as: "Author"})

Adding associations to a relation with a custom join table can be done in two ways (continuing with the associations defined in the previous chapter):

// Either by adding a property with the name of the join table model to the object, before creating the association
project.UserProjects = {
  status: 'active'
// Or by providing a second argument when adding the association, containing the data that should go in the join table
u.addProject(project, { status: 'active' })
// When associating multiple objects, you can combine the two options above. In this case the second argument
// will be treated as a defaults object, that will be used if no data is provided
project1.UserProjects = {
    status: 'inactive'
u.setProjects([project1, project2], { status: 'active' })
// The code above will record inactive for project one, and active for project two in the join table

When getting data on an association that has a custom join table, the data from the join table will be returned as a DAO instance:

u.getProjects().success(function(projects) {
  var project = projects[0]
  if (project.UserProjects.status === 'active') {
    // .. do magic
    // since this is a real DAO instance, you can save it directly after you are done doing magic

If you only need some of the attributes from the join table, you can provide an array with the attributes you want:

// This will select only name from the Projects table, and only status from the UserProjects table
user.getProjects({ attributes: ['name'], joinTableAttributes: ['status']})

Check associations

You can also check if an object is already associated with another one (N:M only). Here is how you'd do it:

// check if an object is one of associated ones:
Project.create({ /* */ }).success(function(project) {
  User.create({ /* */ }).success(function(user) {
    project.hasUser(user).success(function(result) {
      // result would be false
      project.addUser(user).success(function() {
        project.hasUser(user).success(function(result) {
          // result would be true
// check if all associated objects are as expected:
// let's assume we have already a project and two users
project.setUsers([user1, user2]).success(function() {
  project.hasUsers([user1]).success(function(result) {
    // result would be false
    project.hasUsers([user1, user2]).success(function(result) {
      // result would be true

Foreign Keys

When you create associations between your models in sequelize, foreign key references with constraints will automatically be created. The setup below:

var Task = this.sequelize.define('Task', { title: Sequelize.STRING })
  , User = this.sequelize.define('User', { username: Sequelize.STRING })

Will generate the following SQL:

  `username` VARCHAR(255)

  `title` VARCHAR(255), 

The relation between task and user injects the user_id foreign key on tasks, and marks it as a reference to the User table. By default user_id will be set to NULL if the referenced user is deleted, and updated if the id of the user id updated. These options can be overriden by passing onUpdate and onDelete options to the association calls. The validation options are RESTRICT, CASCADE, NO ACTION, SET DEFAULT, SET NULL.

For 1:1 and 1:m associations the default option is SET NULL for deletion, and CASCADE for updates. For n:m, the default for both is CASCADE. This means, that if you delete or update a row from one side of an n:m association, all the rows in the join table refrencing that row will also be deleted or updated.

Adding constriants between tables means that tables must be created in the database in a certain order, when using sequelize.sync. If Task has a reference to User, the User table must be created before the Task table can be created. This can sometimes lead to circular references, where sequelize cannot find an order in which to sync. Imagine a scenario of documents and versions. A document can have multiple versions, and for convenience, a document has an reference to it's current version.

var Document = this.sequelize.define('Document', {
      author: Sequelize.STRING
  , Version = this.sequelize.define('Version', {
      timestamp: Sequelize.DATE

Document.hasMany(Version) // This adds document_id to version
Document.belongsTo(Version, { as: 'Current', foreignKey: 'current_version_id'}) // This adds current_version_id to document

However, the code above will result in the following error: Cyclic dependency found. 'Document' is dependent of itself. Dependency Chain: Document -> Version => Document. In order to alleviate that, we can pass constraints: false to one of the associations:

Document.belongsTo(Version, { as: 'Current', foreignKey: 'current_version_id', constraints: false})

Which will allow us to sync the tables correctly:

  `author` VARCHAR(255), 
  `current_version_id` INTEGER
  `timestamp` DATETIME, 

Enforcing a foreign key reference without constraints

Some times you may want to reference another table, without adding any constraints, or associations. In that case you can manually add the reference attributes to your schema definition, and mark the relations between them.

var Series, Trainer, Video
// Series has a trainer_id=Trainer.id foreign reference key after we call Trainer.hasMany(series)
Series = sequelize.define('Series', {
  title:        DataTypes.STRING,
  sub_title:    DataTypes.STRING,
  description:  DataTypes.TEXT,
  // Set FK relationship (hasMany) with `Trainer`
  trainer_id: {
    type: DataTypes.INTEGER,
    references: "Trainer",
    referencesKey: "id"
Trainer = sequelize.define('Trainer', {
  first_name: DataTypes.STRING,
  last_name:  DataTypes.STRING
// Video has a series_id=Series.id foreign reference key after we call Series.hasOne(Video)...
Video = sequelize.define('Video', {
  title:        DataTypes.STRING,
  sequence:     DataTypes.INTEGER,
  description:  DataTypes.TEXT,
  // set relationship (hasOne) with `Series`
  series_id: {
    type: DataTypes.INTEGER,
    references: Series, // Can be both a string representing the table name, or a reference to the model
    referencesKey: "id"