多态has_many：通过轨道

I have a tags model that I'd like to be polymorphic, but I don't want five records for a tag of "video" for example, I want to create the tag once and be able to use it on a variety of models. I've ready some of the questions here about doing that, but I'm not quite getting how to make it work.I have a tags model that I'd like to be polymor