double default_scope query

Gemfile

# A sample Gemfile
source "https://rubygems.org"

gem 'activerecord', github: 'rails/rails', branch: 'master'
gem 'arel', github: 'rails/arel', branch: 'master'
gem 'pg'

Gemfile.lock

GIT
  remote: git://github.com/rails/arel.git
  revision: d69146d3db96116ef241e3c8b0eb122104bb780b
  branch: master
  specs:
    arel (6.0.0.beta2)

GIT
  remote: git://github.com/rails/rails.git
  revision: 4426b726bfd3f7d26224c2d8ffdc3c00cd1b561a
  branch: master
  specs:
    activemodel (4.2.0.beta4)
      activesupport (= 4.2.0.beta4)
      builder (~> 3.1)
    activerecord (4.2.0.beta4)
      activemodel (= 4.2.0.beta4)
      activesupport (= 4.2.0.beta4)
      arel (>= 6.0.0.beta2, < 6.1)
    activesupport (4.2.0.beta4)
      i18n (>= 0.7.0.beta1, < 0.8)
      json (~> 1.7, >= 1.7.7)
      minitest (~> 5.1)
      thread_safe (~> 0.1)
      tzinfo (~> 1.1)

GEM
  remote: https://rubygems.org/
  specs:
    builder (3.2.2)
    i18n (0.7.0.beta1)
    json (1.8.1)
    minitest (5.4.3)
    pg (0.17.1)
    thread_safe (0.3.4)
    tzinfo (1.2.2)
      thread_safe (~> 0.1)

PLATFORMS
  ruby

DEPENDENCIES
  activerecord!
  arel!
  pg

test.rb

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.establish_connection(
  {
    adapter: "postgresql",
    database: "onruby_development",
    username: "postgres",
    host: "localhost",
    encoding: "utf8",
  }
)

ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts do |t|
    t.string :name
  end

  create_table :comments do |t|
    t.integer :post_id
    t.string :name
  end
end rescue nil

class Post < ActiveRecord::Base
  has_many :comments
  default_scope -> { where(name: 'test') }
end

class Comment < ActiveRecord::Base
  belongs_to :post

  default_scope -> { joins(:post).where("posts.name" => 'test').readonly(false) }
end

class BugTest < Minitest::Test
  def test_association_stuff
    post = Post.create!(name: 'test')
    post.comments << Comment.create!(name: 'test')

    assert_equal post.comments.map(&:id), Post.find(post.id).comments.map(&:id)
  end
end