do you think it's the best solution?

I have:

class Shop < ActiveRecord::Base
has_many :documents, :dependent => :restrict
has_many :roles, :dependent => :restrict
accepts_nested_attributes_for :documents
accepts_nested_attributes_for :roles

the shop _form is:

= simple_form_for(@shop) do |shop_f|
= render 'shared/error_messages', :object => @shop
= render :partial => 'document_fields_form', :locals => { :f =>
shop_f } if @shop.new_record? or @document
= field_set_tag t('shop') do
   .inputs
     = shop_f.input :name
.............

I want to create a new document only if I'm inserting a new shop
record or if @document is defined.
Shop_controller e':

def new
   @shop = Shop.new
   @document = @shop.documents.build
   new!
end

In questo modo quando creo un nuovo Shop creo anche un nuovo Document
relativo, poi...

def edit
   @shop = Shop.find(params[:id])
   @document = @shop.documents.build if params[:dlg114]
   edit!
end

But this way if the document is not valid I can't see document fields
because it isn't a new record and @document is not defined.
Can you suggest a better way to do this?