Today I just spent 3 hours (and before a demo!) trying to figure out
why my polymorphic association fields were breaking.
Turns out someone else put in a patch for this 6 months ago. I'm not
sure if its the same solution I'd use... I'd rather find_or_create
attempt to use AssociationProxy#build if its available and only
Base#new if we're not on a proxy.
Can someone on core look at this. I can do another version if anyone
would prefer. But, I'd love to have this fixed.