sti_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', '~> 5.2.0' # Fails with '~> 5.1.0'
  gem 'sqlite3', '1.3.12'
  gem 'goldiloader'
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(: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!
    article1 = user.articles.create!
    article2 = user.articles.create!
    post = Post.create!(user: user)

    # Explicit eager loading
    articles = Group.includes(:articles).flat_map(&:articles).sort_by(&:id)
    assert_equal([article1, article2], articles)

    # Automatic eager loading
    articles = Group.all.flat_map(&:articles).sort_by(&:id)
    assert_equal([article1, article2], articles)
  end

end