Example flow is on the image. Here I want to accomplish if playlist removed, make it delete image too.

I know I should've put FK on image table but Image is generic and its used more than once.

What are my options? Triggers and application-side comes to mind, but I'm not sure. Maybe there is a better way.

UPDATE: I guess I've found what I was looking for, rules:

CREATE RULE playlist_delete AS ON DELETE TO playlist WHERE (SELECT id FROM image WHERE image.id = OLD.image_id) IS NOT NULL DO INSTEAD NOTHING;