Reordering without explicit direction doesn't work with last in Rails 4

[owst]

This might be an arel bug rather than a baby_squeel bug, apologies if so!

Issue

When using Active Record 4,

Foo.reorder(:x).last

appears to not be equivalent to

Foo.reordering { x }.last

but it is with Active Record 5.

Specifically, it appears that we don't get a DESC in the resulting query in AR 4:

Foo Load (0.1ms)  SELECT  "foos".* FROM "foos"  ORDER BY "foos"."x" LIMIT 1

whereas we do in AR 5:

 Foo Load (0.1ms)  SELECT  "foos".* FROM "foos" ORDER BY "foos"."x" DESC LIMIT ?  [["LIMIT", 1]]

N.B. adding an explicit direction does work in both AR versions:

Foo.reorder('x ASC').last == Foo.reordering { x.asc }.last

Reproduction

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'
  gem 'activerecord', '= 4.2.10' # breaks
  # gem 'activerecord', '= 5.1.4' # works
  gem 'sqlite3'
  gem 'baby_squeel', github: 'rzane/baby_squeel'
end

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

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

ActiveRecord::Schema.define do
  create_table :foos do |t|
    t.integer :x
  end
end

class Foo < ActiveRecord::Base
end

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

class BabySqueelTest < Minitest::Spec
  it 'works' do
    one = Foo.create(x: 1)
    two = Foo.create(x: 2)

    puts "reorder"
    assert_equal one, Foo.reorder('x ASC').first
    assert_equal one, Foo.reorder('x DESC').last
    assert_equal one, Foo.reorder('x').first
    assert_equal two, Foo.reorder('x ASC').last
    assert_equal two, Foo.reorder('x DESC').first
    assert_equal two, Foo.reorder('x').last

    puts "reordering"
    assert_equal one, Foo.reordering { x.asc }.first
    assert_equal one, Foo.reordering { x.desc }.last
    assert_equal one, Foo.reordering { x }.first
    assert_equal two, Foo.reordering { x.asc }.last
    assert_equal two, Foo.reordering { x.desc }.first
    assert_equal two, Foo.reordering { x }.last # Doesn't work with AR 4
  end
end

[bhaak]

Just to clarify, this not only affects reordering but also ordering.