Ah ha! I see where I was going wrong now...
I had the has_many :child_categories relationship set up (I was using
the class_name parameter on that one), and then I had a
belongs_to :parent_category set up also. I wasn't thinking that
getting to the parent was already possible, and once I removed the
belongs_to it migrated.
Thanks Kevin. Big help to me as I would've quit with this strategy and
gone with something else immediately.
Anyone else got suggestions for menu/navigation elements?