Question regarding has_one and association_build

In the doc, there's is a note saying association_build "only works if an association already exists. It will NOT work if the association is nil." But in my test, it works fine when the association is nil. For example,

class Post < ActiveRecord::Base   has_one :author

  def after_initialize       build_author   end end

This works perfectly for me. So is the warning in the doc something I should be worried about?

Link to doc: ActiveRecord::Associations::ClassMethods

Thanks, Eric