Retrieve all models from polymorphic many-to-many relation

I want to retrieve all models involved in polymorphic many-to-many relation. Basing on example given in docs: http://ift.tt/2geTTrV I want to retrieve posts() and videos() in one commentable() relation.

I know that I can just merge two collections but then I'll have to edit that every time when I'm adding new type of commentable - and I want to avoid that.

via Chebli Mohamed