ThroughAssociation using same table twice (STI type and default_scope condition) problem

edge.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    gem 'sqlite3'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

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.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :people, force: true do |t|
    t.string :type
    t.boolean :active, default: true
    t.integer :parent_id
  end

  create_table :toys, force: true do |t|
    t.integer :child_id
  end
end

class Person < ActiveRecord::Base
end

class Parent < Person
end

class Child < Person
  default_scope { where(active: true) }
  belongs_to :parent
end

class Toy < ActiveRecord::Base
  belongs_to :child
  has_one :parent, through: :child
end

class BugTest < Minitest::Test

  def test_has_one_through_sti_1
    # should get parent, but doesn't (in Rails 4.2) because STI type child is incorrectly
    # applied to `people` table instead of `children_parent_join` table in query
    # Fails in Rails 4.2
    # Passes in Rails master, but probably only due to a side effect of something else

    parent = Parent.create!
    child = Child.create!(parent: parent)
    toy = Toy.create!(child: child)

    assert_equal toy.parent, parent
  end

  def test_has_one_through_sti_2
    # should get parent, but doesn't because default_scope active
    # is incorrectly applied to `people` table insetad of `children_parent_join` table in query
    # Fails in Rails 4.2 and master

    parent = Parent.create!(active: false)
    child = Child.create!(parent: parent)
    toy = Toy.create!(child: child)

    assert_equal toy.parent, parent
  end

  def test_has_one_through_sti_3
    # should not get parent because child association is nil (due to default scope active)
    # but the query puts the default scope on the wrong table, so child is found even though he is inactive
    # and his parent is found too.
    #
    # Passes in Rails 4.2 (for the wrong reasons)
    # Fails in master

    parent = Parent.create!
    child = Child.create!(parent: parent, active: false)
    toy = Toy.create!(child: child)

    toy.reload

    assert_nil toy.child
    assert_nil toy.parent
  end

end

gem.rb

gem 'activerecord', '4.2.0'

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.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :people, force: true do |t|
    t.string :type
    t.boolean :active, default: true
    t.integer :parent_id
  end

  create_table :toys, force: true do |t|
    t.integer :child_id
  end
end

class Person < ActiveRecord::Base
end

class Parent < Person
end

class Child < Person
  default_scope { where(active: true) }
  belongs_to :parent
end

class Toy < ActiveRecord::Base
  belongs_to :child
  has_one :parent, through: :child
end

class BugTest < Minitest::Test

  def test_has_one_through_sti_1
    # should get parent, but doesn't (in Rails 4.2) because STI type child is incorrectly
    # applied to `people` table instead of `children_parent_join` table in query
    # Fails in Rails 4.2
    # Passes in Rails master, but probably only due to a side effect of something else

    parent = Parent.create!
    child = Child.create!(parent: parent)
    toy = Toy.create!(child: child)

    assert_equal toy.parent, parent
  end

  def test_has_one_through_sti_2
    # should get parent, but doesn't because default_scope active
    # is incorrectly applied to `people` table insetad of `children_parent_join` table in query
    # Fails in Rails 4.2 and master

    parent = Parent.create!(active: false)
    child = Child.create!(parent: parent)
    toy = Toy.create!(child: child)

    assert_equal toy.parent, parent
  end

  def test_has_one_through_sti_3
    # should not get parent because child association is nil (due to default scope active)
    # but the query puts the default scope on the wrong table, so child is found even though he is inactive
    # and his parent is found too.
    #
    # Passes in Rails 4.2 (for the wrong reasons)
    # Fails in master

    parent = Parent.create!
    child = Child.create!(parent: parent, active: false)
    toy = Toy.create!(child: child)

    toy.reload

    assert_nil toy.child
    assert_nil toy.parent
  end

end