Dynamically setting Paperclip processors with attachment instance data

I'm trying to dynamically set my Paperclip processors which are stored
in my database, ideally I would want to access
attachment.instance.paperclip_processor, but I can't seem to do that.
Any ideas?

# class Post < ActiveRecord::Base
has_attached_file :image,
  :path => ":rails_root/
public/:hash_path/:style/:basename.:extension",
  :processors => lambda { |attachment|
attachment.instance.paperclip_processor }
  :styles => {
    :thumb => ["175x175#", :jpg],
    :resized => ["800x800>", :jpg],
  }