Has Many through, dependent: :destroy

has_many_through.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'arel', github: 'rails/arel', branch: 'master'
    gem 'sqlite3'
    gem 'minitest'
  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 :themes do |t|
  end

  create_table :templates do |t|
    t.references :theme
  end

  create_table :page_contents do |t|
    t.references :component, polymorphic: true
    t.references :container, polymorphic: true
  end

  create_table :placeholders do |t|
  end
end

class Theme < ActiveRecord::Base
  has_many :templates, dependent: :destroy
end

class Template < ActiveRecord::Base
  belongs_to :theme

  has_many :page_contents, as: :container, dependent: :destroy
  has_many :placeholders, through: :page_contents, source: :component, source_type: 'Placeholder'
end

class PageContent < ActiveRecord::Base
  # If we uncomment the dependent option, then everything works fine.
  belongs_to :component, polymorphic: true#, dependent: :destroy
  belongs_to :container, polymorphic: true
end

class Placeholder < ActiveRecord::Base
  has_one :page_component, as: :component, class_name: 'PageContent'
  has_one :page_container, as: :container, class_name: 'PageContent'
end


class BugTest < Minitest::Test
  def test_association_stuff
    theme = Theme.create!
    template = theme.templates.create!
    placeholder = template.placeholders.create!

    theme.reload
    template.reload
    placeholder.reload

    theme.destroy
    assert_equal 0, Placeholder.count
  end
end