Polymorphic foreign keys

[Anahkiasen]

Is it possible to show polymorphic foreign keys of that type, as done by Rails (Ruby) or Laravel (PHP) ?

column	type
illustrable_type	string
illustrable_id	integer

I'm assuming this is not possible because SQL doesn't have any structure for it but as DBPatterns is more of a "representation" tool to me it'd seem logical to be allow to "show" that, maybe by specifying a list of entities in the foreign key field, like Book,Author or something.