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,//belongsToManychild.getParents,child.setParents,child.createParents,

You can also have multiple relationships

// a parent can have many childrenParent.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' });