gistfile1.rb

module DropInTheBucket
  class File < ::ActiveRecord::Base
    belongs_to :dropinable, polymorphic: true
    mount_uploader :item, PhotoUploader
  end
end

---

class Note < ActiveRecord::Base
  has_dropin :photo
end

---

drop_in_the_bucket/orm/active_record/file.rb:4:in `<class:File>': uninitialized constant DropInTheBucket::File::PhotoUploader (NameError)