association build changed under rails 2.1?

class Post < ActiveRecord::Base
has_many :assets

This used to work fine

but under 2.1 it works sometimes and not others. Don't know if that
is because Asset uses attachment_fu (which I've updated to latest)

But this is okay under 2.1

self.assets <<

I'm probably doing something wrong. Which way is, well, wrong_er_?