Rails: What's the best way to handle my lists?

Ok so I'm having a slight problem building my CMS for a client. I want to utilize the same model to handle the photos for their case studies as well as their services. Each has their own model. So the Photo model belongs to a Project or a Service. But in reality it belongs to a project OR a service. Thus I have a problem using acts_as_list.

class Photo < ActiveRecord::Base
  belongs_to :service
  belongs_to :project

  acts_as_list :scope => :project_id

  file_column :image, :magick => { :versions => { "thumb" => "50x50", "medium" => "640x480>" } }

If I could do something like:

acts_as_list :scope => :project_id || :service_id

Then this would be a piece of cake but it's not that easy. How would you solve this? I know it's not that challenging.