Preloading associations with ActiveRecord::Associations::Preloader

The other day I was working to reduce the number of N+1 queries for different pages in a rails app. I found I needed to preload some associations for an object similar to current_user, so for this example we will just use current user. The current_user helper method is defined in the ApplicationController all other controllers that inherit from ApplicationController and their views have access to current_user. Most of the time we don’t need associations to be preloaded when working with current_user, but sometimes we do. Let’s say we want to get all of the current_user’s posts and all of the comments associated with each of the posts. If we try to override current_user to use includes it won’t work, we’ll get a no method error for includes on the instance of User.

# Will raise no method error since .includes is a class methtod
class UserPostsController < ApplicationController
  def index
    @posts = current_user.posts
  end
  
  private

  def current_user
    super.includes(posts: :comments)
  end
end

A better way I found is to use ActiveRecord::Associations::Preloader. Here we can pass records and the associations to preload for the records that were passed. Now there will only be a total of 3 queries for this page, one for the user, one for the user’s posts, and one for the post’s comments.

class UserPostsController < ApplicationController
  def index
    @posts = current_user.posts
  end

  private

  def current_user
    super.tap do |user|
      ActiveRecord::AssociationsPreloader.new(
        records: Array(user),
        associations: { posts: :comments }
      ).call
    end
  end
end