If you're building a web application using Ruby on Rails, you might find that your queries are becoming slow as the size of your database grows. One way to improve performance is to use eager loading, which allows you to load associated records upfront instead of making separate queries for each record.

Table of contents

1. Eager Loading
2. Lazy Loading
3. Preloading
4. Include vs Joins

Eager Loading

Eager loading is a technique that allows you to load associated records in a single query instead of querying the database multiple times. This can significantly improve the performance of your application by reducing the number of database calls.

For example, let's say you have a User model that has_many posts. If you were to retrieve all the users and their associated posts, you could use eager loading like this:

@users = User.includes(:posts)

This will retrieve all the users and their associated posts in a single query, rather than making a separate query for each user's posts.

Other Types of Loading

In addition to eager loading, Rails also provides other types of loading that you can use to optimize your queries:

Lazy Loading

Lazy loading is the opposite of eager loading. Instead of loading associated records upfront, lazy loading loads them when they are needed. This can be useful when you have a large number of associations and don't want to load them all at once.

@user = User.find(1)
@posts = @user.posts # Posts are loaded when called

Preloading

Preloading is similar to eager loading, but instead of joining the associated records in a single query, it makes a separate query for each association. This can be useful when you need to modify the association's query, or when the associated records are not needed in every query.

@users = User.preload(:posts)

Include vs Joins

Both includes and joins can be used for eager loading, but they behave differently, .includes will load the associated records using separate queries, while joins will join the associated records into a single query.

# Includes
@users = User.includes(:posts)

# Joins
@users = User.joins(:posts)

Use includes when you don't need the associated records in every query, and joins when you do.

Conclusion

Optimizing database queries is an important part of building high-performance web applications. Eager loading, lazy loading, preloading, and other types of loading can help you achieve this goal. By understanding these techniques and when to use them, you can improve the performance of your Ruby on Rails application.