Getting Started With Sequelize

4 years ago -  

Sequelize is an Object Relational Mapper (ORM) which enables us to query and manipulate data stored in a relational database. There are several ORM for Node.js out there. Sequelize is one of them and it’s pretty popular. In this short tutorial we will have a quick look at Sequelize then we will write a simple Node.js application to connect to a MySQL database.

Setting up the project

To start off, head over to command line and type the following commands to initialise the project:

take sequelize_todo
yarn init -y
yarn add sequelize mysql2 dotenv ts-node typescript -D

Next, create a file called dbConnection.ts in the root of the project:

import { Sequelize } from "sequelize";

const sequelize = new Sequelize({
  dialect: "mysql",
  username: process.env.DATABASE_USERNAME,
  password: process.env.DATABASE_PASSWORD,
  database: process.env.DATABASE_NAME,
  port: +process.env.DATABASE_PORT || 3306,

export const database = {

As you can see we have constructed an object using Sequelize constructor. This class needs some options which are required to connect to our MySQL database (that’s why we have set dialect to “mysql”. You can changes that to connect to other relational databases like SQL server, …). Then we exported the newly created object.

Next, create an index.ts file in the root of the project:

import * as dotenv from "dotenv";
import { database } from "./dbConnection";
(async () => {
  await database.sequelize.sync();

The first couple of lines are used to load .env file into process.env. This means that we will need to create another file in the root of the project to define our secret values:

DATABASE_HOST = localhost;

NOTE: Make sure you gitignore the .env file before pushing the code into remote repository.

Creating your first entity

Now we need to define our entity which is an abstraction that represents a table in your database. An entity is just a class that extends Model. So let’s create a directory called models then create a file called User.ts inside that directory:

import * as Sequelize from "sequelize";

class User extends Sequelize.Model {
  username: string;
  createdAt: Date;
export const InitUser = (sequelize: Sequelize.Sequelize) => {
      username: Sequelize.DataTypes.STRING,
      createdAt: Sequelize.DataTypes.DATE,
      tableName: "Users",

To define our entity and its properties (Columns) we need to have access to the sequelize object which we exported previously. So let’s head over to the dbConnection.ts file and update the file by passing sequelize object to the InitUser (which we have just exported):

export const database = {
  User: InitUser(sequelize),

Now if you run the project ts-node index.ts you will get this error:

(node:11377) UnhandledPromiseRejectionWarning: SequelizeConnectionError: Unknown database 'sequelize_sample'

It’s because Sequelize doesn’t know how to created the database so it needs a database to start with. So let’s create the schema in MySQL:

CREATE SCHEMA `sequelize_sample` ;

Now if you run the project one more time, Sequelize is able to create the table for you:

Executing (default): CREATE TABLE IF NOT EXISTS `Users` (`id` INTEGER NOT NULL auto_increment , `username` VARCHAR(255), `createdAt` DATETIME, `updatedAt` DATETIME NOT NULL, PRIMARY KEY (`id`)) ENGINE=InnoDB;
Executing (default): SHOW FULL COLUMNS FROM `Users`;
Executing (default): SELECT CONSTRAINT_NAME as constraint_name,CONSTRAINT_NAME as constraintName,CONSTRAINT_SCHEMA as constraintSchema,CONSTRAINT_SCHEMA as constraintCatalog,TABLE_NAME as tableName,TABLE_SCHEMA as tableSchema,TABLE_SCHEMA as tableCatalog,COLUMN_NAME as columnName,REFERENCED_TABLE_SCHEMA as referencedTableSchema,REFERENCED_TABLE_SCHEMA as referencedTableCatalog,REFERENCED_TABLE_NAME as referencedTableName,REFERENCED_COLUMN_NAME as referencedColumnName FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE where TABLE_NAME = 'Users' AND CONSTRAINT_NAME!='PRIMARY' AND CONSTRAINT_SCHEMA='sequelize_sample' AND REFERENCED_TABLE_NAME IS NOT NULL;
Executing (default): ALTER TABLE `Users` CHANGE `username` `username` VARCHAR(255);
Executing (default): ALTER TABLE `Users` CHANGE `createdAt` `createdAt` DATETIME;
Executing (default): ALTER TABLE `Users` CHANGE `updatedAt` `updatedAt` DATETIME NOT NULL;
Executing (default): SHOW INDEX FROM `Users` FROM `sequelize_sample`

In the next blog post I will explain how to query and manipulate data. You can find the code here on GitHub. (