Nested has_many throughs with default scopes don't work as expected

ar_master_test.rb

unless File.exists?('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'

# 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 :topics do |t|
  end

  create_table :conversations do |t|
    t.integer :topic_id
    t.integer :parent_id
  end
end

class Topic < ActiveRecord::Base
  has_many :conversations
  has_many :replies, through: :conversations
end

class Conversation < ActiveRecord::Base
  belongs_to :topic
  has_many :replies, foreign_key: "parent_id"

  def self.default_scope
    where(parent_id: nil)
  end
end

class Reply < Conversation
  belongs_to :parent, class_name: "Conversation"

  def self.default_scope
    where(arel_table[:parent_id].not_eq(nil))
  end
end


class BugTest < MiniTest::Unit::TestCase
  def test_association_stuff
    topic = Topic.create
    conversation = Conversation.create(topic: topic)
    reply = Reply.create(parent: conversation, topic: topic)

    assert_equal([reply], topic.replies)
  end
end