creating an object with its associated.

I have a Shop and a Document model. Shop has_many :documents and Document belongs_to :shop. I want to create a document associated to a shop. routes.rb has: resources :shops do     resources :documents end

In the document form I've set:

= f.input :shop_id, :as => :hidden if @document.new_record?

I'm using simple_form. When I submit the form the document is created but is not associated to the shop. It works if I do @document = Document.new(:shop_id => params[:shop_id]) in the new action. Why I have to specify shop_id attribute? shop_id is a document attribute so I think that I do not specify it on document creation.