rails_issue_11078.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'sqlite3'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

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(:groups) do |t|
  end

  ActiveRecord::Base.connection.create_table(:users) do |t|
    t.integer :group_id
  end

  ActiveRecord::Base.connection.create_table(:posts) do |t|
    t.integer :user_id
    t.string :type
  end
end

# Models
class Group < ActiveRecord::Base
  has_many :users
  has_many :articles, through: :users
end

class User < ActiveRecord::Base
  belongs_to :group
  has_many :articles
end

class Post < ActiveRecord::Base
  belongs_to :user
end

class Article < Post
end

class BugTest < Minitest::Test

  def test_bug
    group = Group.create!
    user = group.users.create!
    article = Article.create!(user: user)
    post = Post.create!(user: user)

    # Works without eager loading
    groups = Group.all.to_a
    assert_equal([article], groups.flat_map(&:articles))

    # Fails with eager loading
    groups = Group.includes(:articles).to_a
    assert_equal([article], groups.flat_map(&:articles))
  end

end