ar_bug.rb

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

    gem 'rails', github: 'rails/rails'
    gem 'arel',  github: 'rails/arel'
    gem 'sqlite3'
  GEMFILE

  system 'bundle --quiet'
end

require 'bundler'
Bundler.setup(:default)

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

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

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts

  create_table :slugs do |t|
    t.string :name
    t.integer :sluggable_id
    t.string :sluggable_type
  end
end

class Slug < ActiveRecord::Base
end

class Post < ActiveRecord::Base
  has_one :slug, as: :sluggable

  scope :by_slug, ->(slug) { joins(:slug).where(slugs: { name: slug }) }
end

class BugTest < Minitest::Test
  def setup
    Post.delete_all
    Slug.delete_all

    @post = Post.create!(slug: Slug.new(name: 'the-slug'))
    @relation = Post.by_slug('the-slug')
  end

  def test_polymorphic_count
    assert_equal 1, @relation.count
  end

  def test_polymorphic_load
    assert_equal [@post], @relation.to_a
  end

  def test_polymorphic_exists
    assert_equal true, @relation.exists?
  end
end