I struggle to find a model how to store a playlist with different type of items on it in Rails. Consider I have class Track end class Album has_many :tracks end class PlaylistItem belongs_to :playable belongs_to :playlist end class Playable belongs_to :playable, :polymorph => true end class Playlist has_many :playlist_items end I think I can use a polymorphic model "Playable" here since the Playlist can contain Tracks, Albums and maybe in the future also Movies. Also I would like to use STI for Track and Albums since they share some common attributes like title and length but also have totally different attributes. I modeled it like described here but it does not work. Anybody any idea how to model a Playlist that can contain many items of different kind?