Rathole, a fixtures plugin so sweet it’s been incorporated into edge rails, is great—but unfortunately it doesn’t support polymorphic associations. That is, until now! :)

Get the patch: here.

UPDATE: I learned that the author, John Burnette, has renamed the project to foxy fixtures. He also massaged my patch into a contribution for rails 2.0: Ticket 10183.

There is some discussion that this patch is not necessary.