Audio Subjects exist to link the Audio class with a variety of classes which are featured as subjects of Audio files.

schema.rb snippet:

  create_table "audio_subjects", :force => true do |t|
    t.column "audio_id",     :integer, :null => false
    t.column "subject_id",   :integer, :null => false
    t.column "subject_type", :string,  :null => false
  end