spec.rb

begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(false) do
  source 'https://rubygems.org'

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem 'activerecord', '6.1.3'
  gem 'activestorage', '6.1.3'
  gem 'rails', '6.1.3'
  gem 'pg', '>= 0.18', '< 2.0'
  gem 'rspec'
  gem 'pry'
end

require 'active_record'
require 'active_storage'
require 'rspec'

ActiveRecord::Tasks::DatabaseTasks.drop('adapter' => 'postgresql', 'database' => 'items-test')
ActiveRecord::Tasks::DatabaseTasks.create('adapter' => 'postgresql', 'database' => 'items-test')
ActiveRecord::Base.establish_connection('adapter' => 'postgresql', 'database' => 'items-test')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :categories do |t|
    t.timestamps
  end

  create_table :items do |t|
    t.belongs_to :category
    t.timestamps
  end
end

class Category < ActiveRecord::Base
  has_many :items
end

class Item < ActiveRecord::Base
  belongs_to :category

  scope :five_per_category, -> {
    from('(SELECT *, ROW_NUMBER() OVER (PARTITION BY category_id ORDER BY category_id) FROM items) AS items').where('row_number <= 5')
  }
end

RSpec.describe do
  before do
    category_1 = Category.create!
    category_2 = Category.create!

    4.times { category_1.items.create! }
    6.times { category_2.items.create! }
  end

  it 'preloading works, nice' do
    categories = Category.all
    items = Item.five_per_category.where(category: categories)
    ActiveRecord::Associations::Preloader.new.preload(categories, :items, items)

    expect(categories[0].items.size).to eq(4)
    expect(categories[1].items.size).to eq(5)
  end
end

RSpec::Core::Runner.invoke