In this tutorial you will learn to make a simple setup of Sequelize to learn the basics.

Installing

Sequelize is available via npm (or yarn).

npm install --save sequelize

You'll also have to manually install the driver for your database of choice:

# One of the following:
$ npm install --save pg pg-hstore # Postgres
$ npm install --save mysql2
$ npm install --save mariadb
$ npm install --save sqlite3
$ npm install --save tedious # Microsoft SQL Server

Setting up a connection

To connect to the database, you must create a Sequelize instance. This can be done by either passing the connection parameters separately to the Sequelize constructor or by passing a single connection URI:

const Sequelize = require('sequelize');

// Option 1: Passing parameters separately
const sequelize = new Sequelize('database', 'username', 'password', {
  host: 'localhost',
  dialect: /* one of 'mysql' | 'mariadb' | 'postgres' | 'mssql' */
});

// Option 2: Passing a connection URI
const sequelize = new Sequelize('postgres://user:[email protected]:5432/dbname');

The Sequelize constructor takes a whole slew of options that are documented in the API Reference for the Sequelize constructor.

Note: setting up SQLite

If you're using SQLite, you should use the following instead:

const sequelize = new Sequelize({
  dialect: 'sqlite',
  storage: 'path/to/database.sqlite'
});

Note: connection pool (production)

If you're connecting to the database from a single process, you should create only one Sequelize instance. Sequelize will set up a connection pool on initialization. This connection pool can be configured through the constructor's options parameter (using options.pool), as is shown in the following example:

const sequelize = new Sequelize(/* ... */, {
  // ...
  pool: {
    max: 5,
    min: 0,
    acquire: 30000,
    idle: 10000
  }
});

Learn more in the API Reference for the Sequelize constructor. If you're connecting to the database from multiple processes, you'll have to create one instance per process, but each instance should have a maximum connection pool size of such that the total maximum size is respected. For example, if you want a max connection pool size of 90 and you have three processes, the Sequelize instance of each process should have a max connection pool size of 30.

Testing the connection

You can use the .authenticate() function to test if the connection is OK:

sequelize
  .authenticate()
  .then(() => {
    console.log('Connection has been established successfully.');
  })
  .catch(err => {
    console.error('Unable to connect to the database:', err);
  });

Closing the connection

Sequelize will keep the connection open by default, and use the same connection for all queries. If you need to close the connection, call sequelize.close() (which is asynchronous and returns a Promise).

Modeling a table

A model is a class that extends Sequelize.Model. Models can be defined in two equivalent ways. The first, with Sequelize.Model.init(attributes, options):

const Model = Sequelize.Model;
class User extends Model {}
User.init({
  // attributes
  firstName: {
    type: Sequelize.STRING,
    allowNull: false
  },
  lastName: {
    type: Sequelize.STRING
    // allowNull defaults to true
  }
}, {
  sequelize,
  modelName: 'user'
  // options
});

Alternatively, using sequelize.define:

const User = sequelize.define('user', {
  // attributes
  firstName: {
    type: Sequelize.STRING,
    allowNull: false
  },
  lastName: {
    type: Sequelize.STRING
    // allowNull defaults to true
  }
}, {
  // options
});

Internally, sequelize.define calls Model.init.

The above code tells Sequelize to expect a table named users in the database with the fields firstName and lastName. The table name is automatically pluralized by default (a library called inflection is used under the hood to do this). This behavior can be stopped for a specific model by using the freezeTableName: true option, or for all models by using the define option from the Sequelize constructor.

Sequelize also defines by default the fields id (primary key), createdAt and updatedAt to every model. This behavior can also be changed, of course (check the API Reference to learn more about the available options).

Changing the default model options

The Sequelize constructor takes a define option which will change the default options for all defined models.

const sequelize = new Sequelize(connectionURI, {
  define: {
    // The `timestamps` field specify whether or not the `createdAt` and `updatedAt` fields will be created.
    // This was true by default, but now is false by default
    timestamps: false
  }
});

// Here `timestamps` will be false, so the `createdAt` and `updatedAt` fields will not be created.
class Foo extends Model {}
Foo.init({ /* ... */ }, { sequelize });

// Here `timestamps` is directly set to true, so the `createdAt` and `updatedAt` fields will be created.
class Bar extends Model {}
Bar.init({ /* ... */ }, { sequelize, timestamps: true });

You can read more about creating models in the Model.init API Reference, or in the sequelize.define API reference.

Synchronizing the model with the database

If you want Sequelize to automatically create the table (or modify it as needed) according to your model definition, you can use the sync method, as follows:

// Note: using `force: true` will drop the table if it already exists
User.sync({ force: true }).then(() => {
  // Now the `users` table in the database corresponds to the model definition
  return User.create({
    firstName: 'John',
    lastName: 'Hancock'
  });
});

Synchronizing all models at once

Instead of calling sync() for every model, you can call sequelize.sync() which will automatically sync all models.

Note for production

In production, you might want to consider using Migrations instead of calling sync() in your code. Learn more in the Migrations guide.

Querying

A few simple queries are shown below:

// Find all users
User.findAll().then(users => {
  console.log("All users:", JSON.stringify(users, null, 4));
});

// Create a new user
User.create({ firstName: "Jane", lastName: "Doe" }).then(jane => {
  console.log("Jane's auto-generated ID:", jane.id);
});

// Delete everyone named "Jane"
User.destroy({
  where: {
    firstName: "Jane"
  }
}).then(() => {
  console.log("Done");
});

// Change everyone without a last name to "Doe"
User.update({ lastName: "Doe" }, {
  where: {
    lastName: null
  }
}).then(() => {
  console.log("Done");
});

Sequelize has a lot of options for querying. You will learn more about those in the next tutorials. It is also possible to make raw SQL queries, if you really need them.

Promises and async/await

As shown above by the extensive usage of .then calls, Sequelize uses Promises extensively. This means that, if your Node version supports it, you can use ES2017 async/await syntax for all asynchronous calls made with Sequelize.

Also, all Sequelize promises are in fact Bluebird promises, so you have the rich Bluebird API to use as well (for example, using finally, tap, tapCatch, map, mapSeries, etc). You can access the Bluebird constructor used internally by Sequelize with Sequelize.Promise, if you want to set any Bluebird specific options.