Sequelize in Node.js: The Ultimate Guide for Beginners (CJS & ESM)

 

 

Imagine you're building a Node.js app that needs to store data. You could write raw SQL queries, but let’s be honest…

• Writing SQL manually is a nightmare!
• Managing relationships? Too much effort!
• Switching between databases? Good luck with that!

Enter Sequelize, the superhero ORM (Object-Relational Mapping) that makes working with databases in Node.js fun, easy, and flexible! 

Today, we’ll cover everything about Sequelize, including:

• What Sequelize is
• How to install and use Sequelize
• Sequelize with both CommonJS (CJS) and ES Modules (ESM)
• CRUD operations (Create, Read, Update, Delete)
• Associations (One-to-One, One-to-Many, Many-to-Many)
• Migrations & Seeders

Let’s get started! 

1. What is Sequelize?

Sequelize is a popular ORM for Node.js that helps you interact with relational databases like:

• PostgreSQL
• MySQL
• MariaDB
• SQLite
• Microsoft SQL Server

Instead of writing raw SQL queries, Sequelize lets you use JavaScript to define models, relationships, and queries in a structured way.

2. Installing Sequelize in Node.js

Before using Sequelize, install the required packages.

Install Sequelize and a Database Driver

npm install sequelize
npm install mysql2  # For MySQL
npm install pg pg-hstore  # For PostgreSQL
npm install sqlite3  # For SQLite
npm install tedious  # For MSSQL

Choose the driver based on your database.

3. Setting Up Sequelize (CJS & ESM)

Using Sequelize with CommonJS (CJS)

In CommonJS (require), initialize Sequelize like this:

const { Sequelize } = require("sequelize");

// Connect to the database
const sequelize = new Sequelize("database_name", "username", "password", {
  host: "localhost",
  dialect: "mysql", // Change to "postgres", "sqlite", "mssql" if needed
});

// Test the connection
sequelize.authenticate()
  .then(() => console.log("Database connected successfully"))
  .catch(err => console.error("Database connection failed", err));

module.exports = sequelize;

Using Sequelize with ES Module (ESM)

In ES Module (import), initialize Sequelize like this:

import { Sequelize } from "sequelize";

const sequelize = new Sequelize("database_name", "username", "password", {
  host: "localhost",
  dialect: "mysql",
});

try {
  await sequelize.authenticate();
  console.log("Database connected successfully");
} catch (error) {
  console.error("Database connection failed", error);
}

export default sequelize;

4. Defining a Model (Table)

A model in Sequelize represents a table in the database.

Example: Create a User Model

const { DataTypes } = require("sequelize");
const sequelize = require("./database"); // Import Sequelize instance

const User = sequelize.define("User", {
  id: {
    type: DataTypes.INTEGER,
    autoIncrement: true,
    primaryKey: true,
  },
  name: {
    type: DataTypes.STRING,
    allowNull: false,
  },
  email: {
    type: DataTypes.STRING,
    allowNull: false,
    unique: true,
  },
  age: {
    type: DataTypes.INTEGER,
    allowNull: true,
  },
});

module.exports = User;

Don't forget to sync the model with the database!

sequelize.sync()
  .then(() => console.log("Database synchronized 🛠"))
  .catch(err => console.error("Sync failed ❌", err));

5. CRUD Operations with Sequelize

Create (Insert Data)

const newUser = await User.create({
  name: "John Doe",
  email: "john@example.com",
  age: 30,
});
console.log("User Created:", newUser.toJSON());

Read (Get Data)

const users = await User.findAll();
console.log("All Users:", users);

Update (Modify Data)

await User.update({ age: 31 }, { where: { email: "john@example.com" } });
console.log("User Updated");

Delete (Remove Data)

await User.destroy({ where: { email: "john@example.com" } });
console.log("User Deleted ❌");

6. Defining Relationships (Associations)

Sequelize supports relationships like:
One-to-One
One-to-Many
Many-to-Many

One-to-Many Example

A User has many Posts.

const Post = sequelize.define("Post", {
  title: DataTypes.STRING,
  content: DataTypes.TEXT,
});

// Define Relationship
User.hasMany(Post);
Post.belongsTo(User);

// Sync Models
sequelize.sync();

Now you can associate posts with users! 

7. Migrations & Seeders (Managing Database Schema)

Sequelize provides migrations to manage database schema changes and seeders to add dummy data.

Install Sequelize CLI

npm install --save-dev sequelize-cli

Initialize Sequelize

npx sequelize-cli init

This creates a migrations/ and seeders/ folder.

Create a Migration

npx sequelize-cli model:generate --name User --attributes name:string,email:string

Run the migration:

npx sequelize-cli db:migrate

Now, your database is structured properly! 

8. Why Use Sequelize?

• No more raw SQL queries – Write JavaScript instead!
• Easily switch between databases – Change dialect: "mysql" to "postgres".
• Built-in relationships and validations.
• Works with both CommonJS and ES Modules.
• Perfect for both small and large projects!

Conclusion: Become a Sequelize Master!

• Sequelize makes database management easy in Node.js.
• Works with MySQL, PostgreSQL, SQLite, and MSSQL.
• Supports CRUD, relationships, migrations, and seeders.
• Can be used with both CJS (require) and ESM (import).

Now, go forth and build awesome apps with Sequelize! 

#HappyCoding!