Joining with scopes

[donjar]

Issue

I cannot join with scopes. As far as I know I am able to do so with Squeel.

Reproduction

require 'bundler/inline'
require 'minitest/spec'
require 'minitest/autorun'

gemfile true do
  source 'https://rubygems.org'
  gem 'activerecord', '~> 4.2.7.1' # which Active Record version?
  gem 'sqlite3'
  gem 'baby_squeel', github: 'rzane/baby_squeel'
end

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')

ActiveRecord::Schema.define do
  create_table :dogs, force: true do |t|
    t.integer :weight
  end
end

class Dog < ActiveRecord::Base
  scope :double_weight, -> { select(:id).select('2 * weight AS double_weight') }
end

class BabySqueelTest < Minitest::Spec
  it 'works' do
    scope = Dog.joining { Dog.double_weight.as(double_weight).on { id = double_weight.id } }

    scope.to_sql.must_equal %{
      SELECT "dogs".* FROM "dogs" INNER JOIN (SELECT "dogs"."id", 2 * weight as double_weight FROM "dogs") double_weight ON "dogs"."id" = "double_weight"."id"
    }.squish
  end
end

This returns:
BabySqueel::NotFoundError: There is no column or association named 'double_weight' for Dog.

If I use Squeel instead:

rails _4.2.7.1_ new try
cd try
echo "gem 'squeel'" >> Gemfile
bundle
bin/rails g model Dog weight:integer
bin/rake db:migrate

Then add scope :double_weight, -> { select(:id).select('(2 * weight) AS double_weight') } into app/models/dog.rb.

Afterwards, run bin/rails c. Running
Dog.joins { Dog.double_weight.as(double_weight).on { id = double_weight.id } } works.