has_one polymorphic is creating circular dependency with parent model

Use this two models:

class Item < ActiveRecord::Base
belongs_to :itemable, :polymorphic => true end

class Product < ActiveRecord::Base
has_one :item, :as => :itemable, :dependent => destroy accepts_nested_attributes_for :item end

now try to:

@product = Product.new(params[:product]) @product.build_item(params[:item]) @product.valid?

And you will get a error lacking the "item.itemable_id"

So, build_item is defining the correct itemable_type, but are not dealing with the id... I expect it to save "course" first, get it's id and them set it on itemable_id and then save "item".