Surprising return from `parent_name`

01_test.rb

require 'active_support'
require 'active_support/core_ext'
require 'active_record'
require 'test/unit'

# Setup

ActiveRecord::Base.establish_connection(
    :adapter => "sqlite3",
    :database  => ":memory:"
)
 
ActiveRecord::Schema.define do
  create_table :users do |table|
    table.column :gender, :string
  end
end

class User < ActiveRecord::Base
  scope :male, -> { where(gender: 'M') }
end

module M
  class N
  end
end

# Test

class ParentNameTest < Test::Unit::TestCase
  def test_module # passes
    assert_parent_name M::N, 'M'
  end

  def test_ar_scope # fails 4.2
    klass = User.male.class
    name = klass.to_s.split('::').first
    assert_parent_name klass, name
  end

  private

  def assert_parent_name(klass, expectation)
    actual = klass.parent_name
    assert expectation == actual,
      "Expected #{klass}.parent_name to be #{expectation.inspect} but found #{actual.inspect}"
  end
end

Gemfile

source "https://rubygems.org"

case ENV['ACTIVE_VERSION']
when '4.2'
  gem 'activerecord', '= 4.2.1'
  gem 'activesupport', '= 4.2.1'
when '4.0'
  gem 'activerecord', '= 4.0.13'
  gem 'activesupport', '= 4.0.13'
else
  fail 'Specify environment variable ACTIVE_VERSION'
end

gem 'test-unit', '1.2.3'
gem 'sqlite3', '1.3.10'

Rakefile

require 'rake'

rule(/4.\d/) do |t|
  bundle = "ACTIVE_VERSION=#{t.name} bundle"
  sh "#{bundle} install && #{bundle} exec ruby 01_test.rb"
end

task default: %w[4.0 4.2]