Active Record's includes method

Introduction

includes is a method in Rails provided by the ActiveRecord that is used to preload the records limiting the count of the SQL queries to the database. This technique of query is also known as eager loading where related child objects are loaded automatically with it’s parent. For e.g

1
2
3
4
5
6
7
8
  class CategoriesController < ApplicationController

  def show
    @category = Category.includes(:articles).limit(5)
  end

end

The above code will fetch the five categories and it’s associated all the articles at once without having to ping the database multiple times.

How It Works

Assume the above models Category and Article have has_many/belongs_to relationship. Setting up the association allows those class to inherit methods from the ActiveRecord::Associations module. One of which is association method which takes a name as an argument(in our case is ‘:articles’). This method will try to fetch the requested association data out of the @association _ cache(generally empty hash) instance variable by assuming as it’s already been loaded into memory. If the association data is not in the @association _ cache, then the association will have to be loaded from which ActiveRecord fetch the data. For e.g

1
  Category.includes(:articles).first.instance_variable_get(:@association_cache).keys

=>[:articles]

Hence, Rails pre-populated the hash with the respective Category’s articles.

In this way, ‘includes’ method preload the @association_cache instance variable, and, when Active Record goes to fetch the data, it is already there and doesn’t have to query the database again.