How to implement many to many association in sequelize

Sequelize Association Cheatsheet

Updated for Sequelize v2/3/4/5

Generally I think the problems are that we are confused about what tables created, and what methods are gained by associations.

Note: Defining foreignKey or cross table name are optional. Sequelize automatically creates it, but defining it allows coders to read the models and find out what the foreign keys/cross table names are, instead of guessing or needing to access the database.

TLDR;

O:O

// foreign key has to be defined on both sides.
Parent.hasOne(Child, {foreignKey: 'Parent_parentId'})
// "Parent_parentId" column will exist in the "belongsTo" table.
Child.belongsTo(Parent, {foreignKey: 'Parent_parentId'})

O:M

Parent.hasMany(Child, {foreignKey: 'Parent_parentId'})
Child.belongsTo(Parent, {foreignKey: 'Parent_parentId'})

N:M

Parent.belongsToMany(
    Child, 
    {
        // this can be string (model name) or a Sequelize Model Object Class
        // through is compulsory since v2
        through: 'Parent_Child',

        // GOTCHA
        // note that this is the Parent's Id, not Child. 
        foreignKey: 'Parent_parentId'
    }
)

/*
The above reads:
"Parents" belongs to many "Children", and is recorded in the "Parent_child" table, using "Parents"'s ID.
*/

Child.belongsToMany(
    Parent, 
    {
        through: 'Parent_Child',

        // GOTCHA
        // note that this is the Child's Id, not Parent.
        foreignKey: 'Child_childId'
    }
)

Why the verbose "Parent_parentId" and not just "parentId"? This is to make it obvious that it's a foreign key that belonged to "Parent". In most cases it's okay to just use the more succinct "parentId".*

Associations gives you 2 functionality: (1) Eager loading and (2) DAO Methods:

1. Include (Eager loading)

DB.Parent.findOne({ 
    where: { id: 1 },
    include: [ DB.Child ]
}).then(parent => {

    // you should get `parent.Child` as an array of children. 

})

2. Methods gained by hasOne(), hasMany() and belongsTo()/belongsToMany()

Associations give the Data Access Object (DAO) methods:

In setting a Parent.hasOne(Child), methods available to parent DAO instance:

DB.Parent.findOne({ where: { id: 1 } }).then(parent => {

    // `parent` is the DAO
    // you can use any of the methods below:
    parent.getChild
    parent.setChild
    parent.addChild
    parent.createChild
    parent.removeChild
    parent.hasChild

})

In setting a Parent.hasMany(Child), methods available to parent DAO instance:

parent.getChildren,
parent.setChildren,
parent.addChild,
parent.addChildren,
parent.createChild,
parent.removeChild,
parent.hasChild,
parent.hasChildren,

In setting a Child.belongsTo(Parent), methods available to child DAO instance:

child.getParent,
child.setParent,
child.createParent,

//belongsToMany
child.getParents,
child.setParents,
child.createParents,

You can also have multiple relationships

// a parent can have many children
Parent.belongsToMany(Child, {
    as: 'Natural',
    through: 'Parent_Child',
    foreignKey: 'Parent_parentId'
})
// a child must at least have 2 parents (natural mother and father)
Child.belongsToMany(Parent, {
    as: 'Natural',
    through: 'Parent_Child',
    foreignKey: 'Child_childId'
})

Parent.belongsToMany(Child, {
    as: 'Foster',
    through: 'Parent_Child',
    foreignKey: 'Parent_parentId'
})

Child.belongsToMany(Parent, {
    as: 'Foster',
    through: 'Parent_Child',
    foreignKey: 'Child_childId'
});

The above will create the Parent_Child cross table, with NaturalId and FosterId.

delete BookArticles model and update relation to:

m.Book.hasMany(m.Article, {through: 'book_articles'});
m.Article.hasMany(m.Books, {through: 'book_articles'});

This is how i solved the similar problem i had two models a user model

var user = sequelize.define('user', {
    name: {
        Sequelize.STRING(255)
    },
    email: {
        type: Sequelize.STRING(255),
        unique: true,
        validate: {
            isEmail: true
        }
    }
});

and a roles model

var Role = sequelize.define('role', {
    name: {
        Sequelize.ENUM('ER', 'ALL', 'DL')
    },
    description: {
        type: Sequelize.TEXT
    }
});

Then i created the union model UserRole

var UserRole = sequelize.define('user_role', {
    id: {
        type: Sequelize.INTEGER,
        primaryKey: true,
        autoIncrement: true
    },
    name: {
        type: Sequelize.ENUM('Admin', 'Staff', 'Customer', 'Owner')
    }
});

Note: you have to explicitly define the id for UserRole otherwise sequelize will use the two foreign keys in this case user_id and role_id as your primary keys.

Then i created the belongs to many relationship as follows

User.belongsToMany(Role, { as: 'Roles', through: { model: UserRole, unique: false }, foreignKey: 'user_id' });
Role.belongsToMany(User, { as: 'Users', through: { model: UserRole, unique: false }, foreignKey: 'role_id' });