This article was written with the help of AI.

When working with Rails, you've likely encountered the N+1 query problem. This occurs when you have a database query that returns N records, and then you loop through those records and execute a separate query for each one. This can result in a large number of queries being executed, which can slow down your application.

Fortunately, Rails provides a solution to this problem in the form of the includes method. The includes method allows you to load associated records in a single query, rather than querying the database for each record individually.

Here's an example of how you can use includes in Rails:

class Post < ApplicationRecord
  belongs_to :author
  has_many :comments
end

class Author < ApplicationRecord
  has_many :posts
end

class Comment < ApplicationRecord
  belongs_to :post
end

In this example, a post belongs to an author and has many comments. Now, let's say we want to display a list of posts and their authors and comments. We could do this with the following code:

@posts = Post.all

@posts.each do |post|
  puts post.title
  puts post.author.name

  post.comments.each do |comment|
    puts comment.body
  end
end

This code will work, but it will result in N+1 queries being executed - one query to load all the posts, and then one query for each post to load its associated author and comments.

To solve this problem, we can use the includes method to preload the associated records:

@posts = Post.includes(:author, :comments).all

@posts.each do |post|
  puts post.title
  puts post.author.name

  post.comments.each do |comment|
    puts comment.body
  end
end

By using includes, we can load all the posts, their associated authors, and their associated comments in a single query, rather than querying the database for each record individually. This can significantly improve the performance of our application, especially when dealing with large datasets.

In addition to belongs_to and has_many associations, includes also supports has_one, has_many :through, has_one :through, has_and_belongs_to_many, and polymorphic associations.

So, the next time you encounter the N+1 query problem in Rails, remember to use the includes method to preload the associated records and improve the performance of your application.