rails update_all does not include joins

gistfile1.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    # gem 'rails', github: 'rails/rails', branch: '4-2-stable'
    # gem 'rails', '4.2.1.rc2'
    # gem 'rails', '4.2.0'
    # gem 'rails', '4.1.9'
    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 :posts, force: true  do |t|
    t.integer :author_id
        t.string :author_stuff
  end

  create_table :authors do |t|
    t.string :stuff
  end
end

class Post < ActiveRecord::Base
  belongs_to :author
end

class Author < ActiveRecord::Base
end

class BugTest < Minitest::Test
  def test_association_stuff
    Post.joins("authors on authors.id = posts.author_id").update_all('author_stuff = authors.stuff')
  end
end