How to get :parent_type into styles Proc

Here we get :parent_type --> works √

 Paperclip.interpolates :parent_type do |attachment, style|
   attachment.instance.uploadable_type.downcase
 end

Here, :parent_type works in :path => but not in Proc, how do I solve this?

 has_attached_file :attachment,
    :path => ":class/:parent_type/:id/:style.:extension",
    :styles => Proc.new { |a| a.instance.set_style(:parent_type) }