eager_loading_bug.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(true) do
  source 'https://rubygems.org'
  gem 'rails', github: 'rails/rails'
  gem 'sqlite3'
end

require 'active_record'
require 'minitest/autorun'
require 'logger'

puts "Using ActiveRecord #{ActiveRecord::VERSION::STRING}"

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

# Schema
ActiveRecord::Schema.define do
  ActiveRecord::Base.connection.create_table(:blogs)

  ActiveRecord::Base.connection.create_table(:posts) do |t|
    t.integer :blog_id
    t.string :tag
  end
end

# Models
class Blog < ActiveRecord::Base
  has_many :posts
  has_many :limited_posts, -> { order(:id).limit(2) }, class_name: 'Post'
  has_many :tag_counts, -> { group(:tag, :blog_id).select('blog_id, tag, count(*) as tag_count') }, class_name: 'Post'
  has_many :offset_posts, -> { offset(1) }, class_name: 'Post'
end

class Post < ActiveRecord::Base
  belongs_to :blog
end

class BugTest < Minitest::Test
  def setup
    Blog.delete_all
    Post.delete_all
  end

  def test_eager_load_association_with_limit
    blog1 = Blog.create!
    blog_1_limited_posts = Array.new(3) { blog1.posts.create! }.take(2)
    blog2 = Blog.create!
    blog_2_limited_posts = Array.new(3) { blog2.posts.create! }.take(2)

    blogs = Blog.includes(:limited_posts).order(:id).to_a
    assert_equal(blog_1_limited_posts, blogs.first.limited_posts)
    assert_equal(blog_2_limited_posts, blogs.second.limited_posts)
  end

  # Fixed in Rails 5.0.x >= 5.0.7
  # Fixed in Rails 5.1.x >= 5.1.5
  # Fixed in Rails 5.2.x
  def test_eager_load_association_with_group
    blog1 = Blog.create!
    Post.create!(blog: blog1, tag: 'a')
    Post.create!(blog: blog1, tag: 'a')
    Post.create!(blog: blog1, tag: 'b')

    blog2 = Blog.create!
    Post.create!(blog: blog2, tag: 'a')

    blogs = Blog.includes(:tag_counts).order(:id).to_a
    assert_equal({'a' => 2, 'b' => 1}, blogs.first.tag_counts.map { |tc| [tc.tag, tc.tag_count] }.to_h)
    assert_equal({'a' => 1}, blogs.second.tag_counts.map { |tc| [tc.tag, tc.tag_count] }.to_h)
  end

  def test_eager_load_association_with_offset
    blog1 = Blog.create!
    blog_1_offset_posts = Array.new(3) { blog1.posts.create! }.drop(1)
    blog2 = Blog.create!
    blog_2_offset_posts = Array.new(3) { blog2.posts.create! }.drop(1)

    blogs = Blog.includes(:offset_posts).order(:id).to_a
    assert_equal(blog_1_offset_posts, blogs.first.offset_posts)
    assert_equal(blog_2_offset_posts, blogs.second.offset_posts)
  end
end