photo.rb paperclip troubleshooting

class Photo < ActiveRecord::Base
  belongs_to :photographable, :polymorphic => true
  has_attached_file :source,
                    :styles => { :normal => ['112x112^', :png] },
                    :default_style => :normal,
                    :path => ":rails_root/public/assets/:class/:attachment/:id/:style/:basename.:extension",
                    :url => "/assets/:class/:attachment/:id/:style/:basename.:extension"
  acts_as_list :scope => :photographable

end