Rails Master: Polymorphic Bug

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'
end

require 'bundler'
Bundler.setup(:default)

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

# 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 :read_marks do |t|
    t.integer :user_id
    t.integer :readable_id
    t.string  :readable_type
  end

  create_table :users do |t|
  end

  create_table :module_videos do |t|
  end
end

module Readable
  def self.include(base)
    base.has_many :read_marks, as: :readable
  end
end

class User < ActiveRecord::Base
  has_many :read_marks
  has_many :module_videos, through: :read_marks,
    source: :readable, source_type: 'ModuleVideo'
end

class ModuleVideo < ActiveRecord::Base
  include Readable

  has_many :read_marks, as: :readable
  has_many :users, through: :read_marks
end

class ReadMark < ActiveRecord::Base
  belongs_to :user
  belongs_to :readable, polymorphic: true
end

class BugTest < Minitest::Test
  def test_association_stuff
    user = User.create
    video = ModuleVideo.create
    ReadMark.create(user: user, readable: video)

    assert_equal user.module_videos.count, 1
  end
end