This section describes the various association types in sequelize. When calling a method such as User.hasOne(Project), we say that the User model (the model that the function is being invoked on) is the source and the Project model (the model being passed as an argument) is the target.

One-To-One associations

One-To-One associations are associations between exactly two models connected by a single foreign key.

BelongsTo

BelongsTo associations are associations where the foreign key for the one-to-one relation exists on the source model.

A simple example would be a Player being part of a Team with the foreign key on the player.

var Player = this.sequelize.define('player', {/* attributes */})
  , Team  = this.sequelize.define('team', {/* attributes */});
Player.belongsTo(Team); // Will add a teamId attribute to Player to hold the primary key value for Team

Foreign keys

By default the foreign key for a belongsTo relation will be generated from the target model name and the target primary key name.

The default casing is camelCase however if the source model is configured with underscored: true the foreignKey will be snake_case.

var User = this.sequelize.define('user', {/* attributes */})
  , Company  = this.sequelize.define('company', {/* attributes */});
User.belongsTo(Company); // Will add companyId to user
var User = this.sequelize.define('user', {/* attributes */}, {underscored: true})
  , Company  = this.sequelize.define('company', {
    uuid: {
      type: Sequelize.UUID,
      primaryKey: true
    }
  });
User.belongsTo(Company); // Will add company_uuid to user

In cases where as has been defined it will be used in place of the target model name.

var User = this.sequelize.define('user', {/* attributes */})
  , UserRole  = this.sequelize.define('userRole', {/* attributes */});
User.belongsTo(UserRole, {as: 'role'}); // Adds roleId to user rather than userRoleId

In all cases the default foreign key can be overwritten with the foreignKey option.When the foreign key option is used, Sequelize will use it as-is:

var User = this.sequelize.define('user', {/* attributes */})
  , Company  = this.sequelize.define('company', {/* attributes */});
User.belongsTo(Company, {foreignKey: 'fk_company'}); // Adds fk_company to User

Target keys

The target key is the column on the target model that the foreign key column on the source model points to. By default the target key for a belongsTo relation will be the target model's primary key. To define a custom column, use the targetKey option.

var User = this.sequelize.define('user', {/* attributes */})
  , Company  = this.sequelize.define('company', {/* attributes */});
User.belongsTo(Company, {foreignKey: 'fk_companyname', targetKey: 'name'}); // Adds fk_companyname to User

HasOne

HasOne associations are associations where the foreign key for the one-to-one relation exists on the target model.

var User = sequelize.define('user', {/* ... */})
var Project = sequelize.define('project', {/* ... */})
// One-way associations
Project.hasOne(User)
/*
  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.
  The foreign key will be placed on the users table.
  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:
Person#setFather
Person#getFather
// If you need to join a table twice you can double join the same table
Team.hasOne(Game, {as: 'HomeTeam', foreignKey : 'homeTeamId'});
Team.hasOne(Game, {as: 'AwayTeam', foreignKey : 'awayTeamId'});
Game.belongsTo(Team);

Even though it is called a HasOne association, for most 1:1 relations you usually want the BelongsTo association since BelongsTo will add the foreignKey on the source where hasOne will add on the target.

Difference between HasOne and BelongsTo

In Sequelize 1:1 relationship can be set using HasOne and BelongsTo. They are suitable for different scenarios. Lets study this difference using an example.

Suppose we have two tables to link Player and Team. Lets define their models.

var Player = this.sequelize.define('player', {/* attributes */})
  , Team  = this.sequelize.define('team', {/* attributes */});

When we link two model in Sequelize we can refer them as pairs of source and target models. Like this

Having Player as the source and Team as the target

Player.belongsTo(Team);
//Or
Player.hasOne(Team);

Having Team as the source and Player as the target

Team.belongsTo(Player);
//Or
Team.hasOne(Player);

HasOne and BelongsTo insert the association key in different models from each other. HasOne inserts the association key in target model whereas BelongsTo inserts the association key in the source model.

Here is an example demonstrating use cases of BelongsTo and HasOne.

var Player = this.sequelize.define('player', {/* attributes */})
  , Coach  = this.sequelize.define('coach', {/* attributes */})
  , Team  = this.sequelize.define('team', {/* attributes */});

Suppose our Player model has information about its team as teamId column. Information about each Team's Coach is stored in the Team model as coachId column. These both scenarios requires different kind of 1:1 relation because foreign key relation is present on different models each time.

When information about association is present in source model we can use belongsTo. In this case Player is suitable for belongsTo because it has teamId column.

Player.belongsTo(Team)  // `teamId` will be added on Player / Source model

When information about association is present in target model we can use hasOne. In this case Coach is suitable for hasOne because Team model store information about its Coach as coachId field.

Coach.hasOne(Team)  // `coachId` will be added on Team / Target model

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 association in the next section:

Sometimes you may need to associate records on different columns, you may use sourceKey option:

var City = sequelize.define('city', { countryCode: Sequelize.STRING });
var Country = sequelize.define('country', { isoCode: Sequelize.STRING });
// Here we can connect countries and cities base on country code
Country.hasMany(City, {foreignKey: 'countryCode', sourceKey: 'isoCode'});
City.belongsTo(Country, {foreignKey: 'countryCode', targetKey: 'isoCode'});

Belongs-To-Many associations

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

Project.belongsToMany(User, {through: 'UserProject'});
User.belongsToMany(Project, {through: 'UserProject'});

This will create a new model called UserProject with the equivalent foreign keys projectId and userId. Whether the attributes are camelcase or not depends on the two models joined by the table (in this case User and Project).

Defining through is required. Sequelize would previously attempt to autogenerate names but that would not always lead to the most logical setups.

This will add methods getUsers, setUsers, addUser,addUsers to Project, and getProjects, setProjects, addProject, and addProjects 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. We will also manually define the foreign keys to use:

User.belongsToMany(Project, { as: 'Tasks', through: 'worker_tasks', foreignKey: 'userId' })
Project.belongsToMany(User, { as: 'Workers', through: 'worker_tasks', foreignKey: 'projectId' })

foreignKey will allow you to set source model key in the through relation.otherKey will allow you to set target model key in the through relation.

User.belongsToMany(Project, { as: 'Tasks', through: 'worker_tasks', foreignKey: 'userId', otherKey: 'projectId'})

Of course you can also define self references with belongsToMany:

Person.belongsToMany(Person, { as: 'Children', through: 'PersonChildren' })
// This will create the table PersonChildren which stores the ids of the objects.

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.belongsToMany(Project, { through: UserProjects })
Project.belongsToMany(User, { through: UserProjects })

To add a new project to a user and set its 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 previously 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
})

With Belongs-To-Many you can query based on through relation and select specific attributes. For example using findAll with through

User.findAll({
  include: [{
    model: Project,
    through: {
      attributes: ['createdAt', 'startedAt', 'finishedAt'],
      where: {completed: true}
    }
  }]
});

Scopes

This section concerns association scopes. For a definition of association scopes vs. scopes on associated models, see Scopes.

Association scopes allow you to place a scope (a set of default attributes for get and create) on the association. Scopes can be placed both on the associated model (the target of the association), and on the through table for n:m relations.

1:m

Assume we have tables Comment, Post and Image. A comment can be associated to either an image or a post via commentable_id and commentable - we say that Post and Image are Commentable

this.Comment = this.sequelize.define('comment', {
  title: Sequelize.STRING,
  commentable: Sequelize.STRING,
  commentable_id: Sequelize.INTEGER
}, {
  instanceMethods: {
    getItem: function() {
      return this['get' + this.get('commentable').substr(0, 1).toUpperCase() + this.get('commentable').substr(1)]();
    }
  }
});
this.Post.hasMany(this.Comment, {
  foreignKey: 'commentable_id',
  constraints: false,
  scope: {
    commentable: 'post'
  }
});
this.Comment.belongsTo(this.Post, {
  foreignKey: 'commentable_id',
  constraints: false,
  as: 'post'
});
this.Image.hasMany(this.Comment, {
  foreignKey: 'commentable_id',
  constraints: false,
  scope: {
    commentable: 'image'
  }
});
this.Comment.belongsTo(this.Image, {
  foreignKey: 'commentable_id',
  constraints: false,
  as: 'image'
});

constraints: false, disables references constraints - since the commentable_id column references several tables, we cannot add a REFERENCES constraint to it. Note that the Image -> Comment and Post -> Comment relations define a scope, commentable: 'image' and commentable: 'post' respectively. This scope is automatically applied when using the association functions:

image.getComments()
SELECT * FROM comments WHERE commentable_id = 42 AND commentable = 'image';
image.createComment({
  title: 'Awesome!'
})
INSERT INTO comments (title, commentable_id, commentable) VALUES ('Awesome!', 42, 'image');
image.addComment(comment);
UPDATE comments SET commentable_id = 42, commentable = 'image'

The getItem utility function on Comment completes the picture - it simply converts the commentable string into a call to either getImage or getPost, providing an abstraction over whether a comment belongs to a post or an image.

n:m

Continuing with the idea of a polymorphic model, consider a tag table - an item can have multiple tags, and a tag can be related to several items.

For brevity, the example only shows a Post model, but in reality Tag would be related to several other models.

ItemTag = sequelize.define('item_tag', {
  tag_id: {
    type: DataTypes.INTEGER,
    unique: 'item_tag_taggable'
  },
  taggable: {
    type: DataTypes.STRING,
    unique: 'item_tag_taggable'
  },
  taggable_id: {
    type: DataTypes.INTEGER,
    unique: 'item_tag_taggable',
    references: null
  }
});
Tag = sequelize.define('tag', {
  name: DataTypes.STRING
});
Post.belongsToMany(Tag, {
  through: {
    model: ItemTag,
    unique: false,
    scope: {
      taggable: 'post'
    }
  },
  foreignKey: 'taggable_id',
  constraints: false
});
Tag.belongsToMany(Post, {
  through: {
    model: ItemTag,
    unique: false
  },
  foreignKey: 'tag_id'
});

Notice that the scoped column (taggable) is now on the through model (ItemTag).

We could also define a more restrictive association, for example to get all pending tags for a post by applying a scope of both the through model (ItemTag) and the target model (Tag):

Post.hasMany(Tag, {
  through: {
    model: ItemTag,
    unique: false,
    scope: {
      taggable: 'post'
    }
  },
  scope: {
    status: 'pending'
  },
  as: 'pendingTags',
  foreignKey: 'taggable_id',
  constraints: false
});
Post.getPendingTags();

SELECT `tag`.*  INNER JOIN `item_tags` AS `item_tag`
ON `tag`.`id` = `item_tag`.`tagId`
  AND `item_tag`.`taggable_id` = 42
  AND `item_tag`.`taggable` = 'post'
WHERE (`tag`.`status` = 'pending');

constraints: false disables references constraints on the taggable_id column. Because the column is polymorphic, we cannot say that it REFERENCES a specific table.

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 associations (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.belongsToMany(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',
  }
})
User.belongsToMany(Project);

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

Remember, that using as to change the name of the association will also change the name of the foreign key. When using as, it is safest to also specify the foreign key.

Invoice.belongsTo(Subscription)
Subscription.hasMany(Invoice)

Without as, this adds subscriptionId as expected. However, if you were to say Invoice.belongsTo(Subscription, { as: 'TheSubscription' }), you will have both subscriptionId and theSubscriptionId, because sequelize is not smart enough to figure that the calls are two sides of the same relation. 'foreignKey' fixes this problem;

Invoice.belongsTo(Subscription, , { as: 'TheSubscription', foreignKey: 'subscription_id' })
Subscription.hasMany(Invoice, { foreignKey: 'subscription_id' )

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:

Project.belongsToMany(Task)
Task.belongsToMany(Project)
Project.create()...
Task.create()...
Task.create()...
// save them... and then:
project.setTasks([task1, task2]).then(function() {
  // saved!
})
// ok, now they are saved... how do I get them later on?
project.getTasks().then(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' }).then(function(tasks) {
  // tasks with an id greater than 10 :)
})
// You can also only retrieve certain fields of a associated object.
project.getTasks({attributes: ['title']}).then(function(tasks) {
    // retrieve tasks with the attributes "title" and "id"
})

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

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

You can of course also do it vice versa:

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

For hasOne/belongsTo its basically the same:

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

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'
}
u.addProject(project)
// 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().then(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
    return project.UserProjects.save()
  }
})

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({ /* */ }).then(function(project) {
  return User.create({ /* */ }).then(function(user) {
    return project.hasUser(user).then(function(result) {
      // result would be false
      return project.addUser(user).then(function() {
        return project.hasUser(user).then(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]).then(function() {
  return project.hasUsers([user1]);
}).then(function(result) {
  // result would be false
  return project.hasUsers([user1, user2]);
}).then(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 })
User.hasMany(Task)
Task.belongsTo(User)

Will generate the following SQL:

CREATE TABLE IF NOT EXISTS `User` (
  `id` INTEGER PRIMARY KEY,
  `username` VARCHAR(255)
);
CREATE TABLE IF NOT EXISTS `Task` (
  `id` INTEGER PRIMARY KEY,
  `title` VARCHAR(255),
  `user_id` INTEGER REFERENCES `User` (`id`) ON DELETE SET NULL ON UPDATE CASCADE
);

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 overridden 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 referencing that row will also be deleted or updated.

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

Which will allow us to sync the tables correctly:

CREATE TABLE IF NOT EXISTS `Document` (
  `id` INTEGER PRIMARY KEY,
  `author` VARCHAR(255),
  `current_version_id` INTEGER
);
CREATE TABLE IF NOT EXISTS `Version` (
  `id` INTEGER PRIMARY KEY,
  `timestamp` DATETIME,
  `document_id` INTEGER REFERENCES `Document` (`id`) ON DELETE SET NULL ON UPDATE CASCADE
);

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: {
      model: "trainers",
      key: "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: {
      model: Series, // Can be both a string representing the table name, or a reference to the model
      key:   "id"
    }
  }
});
Series.hasOne(Video);
Trainer.hasMany(Series);

Creating with associations

An instance can be created with nested association in one step, provided all elements are new.

Creating elements of a "BelongsTo" or "HasOne" association

Consider the following models:

var Product = this.sequelize.define('product', {
  title: Sequelize.STRING
});
var User = this.sequelize.define('user', {
  first_name: Sequelize.STRING,
  last_name: Sequelize.STRING
});
Product.belongsTo(User);
// Also works for `hasOne`

A new Product and User can be created in one step in the following way:

return Product.create({
  title: 'Chair',
  user: {
    first_name: 'Mick',
    last_name: 'Broadstone'
  }
}, {
  include: [ User ]
});

Creating elements of a "BelongsTo" association with an alias

The previous example can be extended to support an association alias.

var Creator = Product.belongsTo(User, {as: 'creator'});
return Product.create({
  title: 'Chair',
  creator: {
    first_name: 'Matt',
    last_name: 'Hansen'
  }
}, {
  include: [ Creator ]
});

Creating elements of a "HasMany" or "BelongsToMany" association

Let's introduce the ability to associate a project with many tags. Setting up the models could look like:

var Tag = this.sequelize.define('tag', {
  name: Sequelize.STRING
});
Product.hasMany(Tag);
// Also works for `belongsToMany`.

Now we can create a project with multiple tags in the following way:

Product.create({
  id: 1,
  title: 'Chair',
  tags: [
    { name: 'Alpha'},
    { name: 'Beta'}
  ]
}, {
  include: [ Tag ]
})

And, we can modify this example to support an alias as well:

var Categories = Product.hasMany(Tag, {as: 'categories'});
Product.create({
  id: 1,
  title: 'Chair',
  categories: [
    {id: 1, name: 'Alpha'},
    {id: 2, name: 'Beta'}
  ]
}, {
  include: [{
    model: Categories,
    as: 'categories'
  }]
})