Looking for Help/Advice on StackOverflow Question - Polymorphic Association with STI

I am having trouble with an many to many polymorphic association with
STI involved. The problem is described clearly in the following
StackOverflow question, so I will leave this thread short:


I am posting here hoping to get advice/direction/a solution for this,
as I've so far been unsuccessful getting help through StackOverflow
itself and #rubyonrails.