acts_as_list and single table inheritence

Hello Rails experts,

Need your help in following issue with the <b>acts_as_list and single
table inheritence</b>

Code details :
#model file
class <b>RelatedItem</b> < ActiveRecord::Base
acts_as_list :scope => :relater
belongs_to :relater, :polymorphic=>true
end

class RelatedLink < <b>RelatedItem</b>

end

class RelatedFeed < <b>RelatedItem</b>

end

class Clip < ActiveRecord::Base
has_many :<b>related_items</b>, :as => :relater
end

## Table Structure of <b>RelatedItem</b>
id :integer(11)
name :string(255)
relater_id :integer(10)
relater_type :string(255)
type :string(255)
position :integer(10)
source :string(255)