polymorphic :dependent => :delete_all fails to delete rows with abstract_class instances

If I have a class hierarchy ContactIndex < Index, GuideIndex < Index, where Index is

  class Index < ActiveRecord::Base
    has_many :links, :as => :link_owner, :dependent => :delete_all
  end

and Link is

  class Link < ActiveRecord::Base
    belongs_to :link_owner, :polymorphic => true
  end

When I instantiate and save a GuideIndex, the links table gets a row with a link_owner_type of Index. When I subsequently delete the index, the links rows are orphaned:

  i = GuideIndex.new
  i << Link.new(:url => 'http://somewhere')
  i.save    # This creates links row with link_owner_type = 'Index'
  i.destroy # This tries to delete rows with link_owner_type = 'GuideIndex' and finds no rows

At present, my workaround is this:

  class Index < ActiveRecord::Base
    has_many :links, :as => :link_owner
    before_destroy :delete_links

  private
    def delete_links
      Link.delete_all ['links.link_owner_id = ? AND links.link_owner_type = ?', self.id, 'Index']
    end
  end