What is Eager Loading in Sequelize.js?

Updated: December 29, 2023 By: Guest Contributor Post a comment


Eager loading is a concept in the context of object-relational mapping (ORM) that refers to the practice of retrieving related data of an entity from the database in a single query, as opposed to lazy loading, where the related data is only loaded when it is specifically requested. Sequelize.js, a popular ORM for Node.js, supports eager loading, which can be immensely beneficial for performance and convenience when dealing with complex data models. This tutorial will explore eager loading in Sequelize.js using practical code examples.

Understanding Eager Loading

In a typical database relationship, you may have a ‘User’ model that has many ‘Posts’. Without eager loading, every time you retrieve a user from the database, you would have to make an additional query to fetch the user’s posts. Eager loading allows you to fetch both the users and their related posts in a single query, thus reducing the number of database calls and potentially improving the performance of your application.

Basic Eager Loading Example

const User = sequelize.define('user', { /* ... */ });
const Post = sequelize.define('post', { /* ... */ });


  include: [{ model: Post }]
}).then(usersWithPosts => {
  // usersWithPosts now contains users as well as their posts

Advanced Eager Loading Techniques

Sequelize.js offers advanced eager loading capabilities such as nested eager loading and eager loading with conditions.

  include: [
      model: Post,
      include: [{ model: Comment }]
}).then(users => {
  // users now contains posts and each post contains comments

  include: [{
    model: Post,
    where: { published: true }
}).then(activeUsers => {
  // activeUsers contains users with only published posts


In summary, eager loading is an effective way to optimize data retrieval in Sequelize.js by minimizing the number of database queries required to fetch related data. Through basic and advanced examples, we have seen how to implement eager loading to improve the performance and maintainability of a Sequelize.js application. As you build more intricate data models and applications, eager loading becomes an increasingly important tool in your optimization arsenal.