Sequelize is an Object-Relational-Mapper, which is written entirely in Javascript and can be used in the Node.JS environment. However, setting it up might seem a little messy since the document does not cover everything that it can do. In this post, I only cover some basic information as well as some tips that you cannot find in the Sequelize Docs.

1. Setting up Sequelize

1.1 Installing Sequelize

In order to use Sequelize, you need to install it along with the database engine of your choice. For example

$ npm install --save sequelize
$ npm install --save pg       # for postgres
$ npm install --save mysql    # for mysql
$ npm install --save sqlite3  # for sqlite
$ npm install --save mariasql # for mariasql

1.2 Connecting to Database

After installing Sequelize, it’s time to test the connection to the database to verify that Sequelize can work properly. This code is taken from Sequelize homepage.

var Sequelize = require('sequelize')
  , sequelize = new Sequelize('database_name', 'username', 'password', {
      dialect: "postgres", // or 'sqlite', mysql', 'mariadb'
      port:    3306, // or 5432 (for postgres)
  .complete(function(err) {
    if (!!err) {
      console.log('Unable to connect to the database:', err)
    } else {
      console.log('Connection has been established successfully.')

2. Defining Database Schema

The Sequelize document describe quite clearly about how to represent your data structure with Sequelize Sequelize Models. You should take a look at the document to know the basic of Sequelize. However, there are something that they didn’t cover in the document. If you want to generate the models from your existing database, head to the next section.

2.1 Composite primary key

To define a composite primary key, simply put the attribute primaryKey: true in each property belongs to that primary key. For example

var myModel = sequelize.define('MyModel', { 
    item1: {
      type: Sequelize.INTEGER,
      allowNull: false,
	  primaryKey: true
    item2: {
      type: Sequelize.INTEGER,
      allowNull: false,
	  primaryKey: true
    item3: {
      type: Sequelize.INTEGER,
      allowNull: false,
	  primaryKey: true

2.2 Foreign key inside model definition

To define a reference for the current model (foreign key), add the attribute references and referencesKey to the property of the model. For example, the Project model has a property userId that references to the id in the User model.

var User = sequelize.define('User', {
	id: {/* */}
	/* other properties */
var Project = sequelize.define('Project', {
	userId: {
		references: "Users", // notice the plural, it's the name of the table
		referencesKey: "id"
	/* other properties */

3. Synchronizing with Database server

3.1 Let Sequelize create the tables automatically

If you don’t have the database structure yet, don’t worry because Sequelize will do it all for you. You only need to define the models and call sequelize.sync(). You can read more here Database Synchronization.

// create all tables... now!
// force it! drop all existing tables and re-create again
sequelize.sync({force: true});

Sometimes, Sequelize cannot drop tables due to the foreign constraints.

Error: ER_ROW_IS_REFERENCED: Cannot delete or update a parent row: a foreign key constraint fails

To fix it, instead of simply call sequelize.sync({force: true});, use this one

sequelize.query('SET FOREIGN_KEY_CHECKS = 0')
    return db.sync({ force: true });
    return db.query('SET FOREIGN_KEY_CHECKS = 1')
    console.log('Database synchronised.');
}, function(err){

3.2 Use with your existing database

By default, Sequelize does not provide any way for you to generate the schema from an existing database. You have to define it manually. However, sequelize-auto can help you partially fulfill this task.

The tool is really easy to use. First, you need to install it using npm

$ npm install -g sequelize-auto

To generate database, execute this command. Change the arguments to your corresponding values.

$ sequelize-auto -o "./output-folder" -d database_name -h localhost -u username -p port -x my_password -e postgres

sequelize-auto will then generate all the models for you in the output-folder. Later, you can use the import function provided by Sequelize (Sequelize.import) to populate your models. However, as I said before, sequelize-auto only reduces the work that you have to do, not a complete solution so you still have to edit the generated model files.

4. Custom column name

By default, Sequelize only allows you to specify custom table name. Currently, there is no such feature in Sequelize to help you define custom column names and then map them to properties in model objects. I have written another post about this problem and the possible workaround here NodeJS - Custom column name in Sequelize.

5. The rest

Again, this post just shows the basic installation, setting up steps that Sequelize Docs does not clearly state. For other features, I advise you to read through the Sequelize Docs.