one form for one model with nested resources.

I have:

Customer
  has_many :deliveries
  has_one :document

Document
  belongs_to :customer

Delivery
  belongs_to :customer

route.rb has:

  resources :customers do
    resources :deliveries
  end

I already have customers in the database, I only have to add
deliveries and document to one customer.
I want to do with one single form.
Obviously this one doesn't work:

<%= form_for([@customer, @delivery]) do |f| %>
  <%= f.input :delivered_at, :as => :hidden, :value => Date.today %>
<%= f.fields_for :document do |doc| %>
  <%= doc.label :doc_type %>
  <%= doc.text_field :doc_type %>
<%= doc.error :doc_type %>
<% end %>
<%= f.submit %>

It adds deliveries but not document.

Some advices?

[http://api.rubyonrails.org/classes/ActiveRecord/NestedAttributes/ClassMethods.html](http://api.rubyonrails.org/classes/ActiveRecord/NestedAttributes/ClassMethods.html)

Haven't tested it, but perhaps you try something like this:
class Customer
has_many :deliveries
has_one :document
accepts_nested_attributes_for :deliveries, :reject_if => lamda { |attributes| attributes['delivered_at'].blank?   }
accepts_nested_attributes_for :document
...
end
  <%= form_for(@customer) do |f| %>
<%= f.fields_for :deliveries, @delivery do |delivery_fields %>
<%= delivery_fields.input :delivered_at, :as => :hidden, :value => Date.today %>
<% end %>
<%= f.fields_for :document do |doc_fields| %>
<%= doc_fields.label :doc_type %>
<%= doc_fields.text_field :doc_type %>
<%= doc_fields.error :doc_type %>
<% end %>
<%= f.submit %>
<% end %>

I've solved in this manner:

<%= form_for([@customer, @delivery]) do |f| %>
<%= f.input :delivered_at, :as => :hidden, :value => Date.today %>
<%= fields_for @document do |doc| %>
<%= doc.label :doc_type %>
<%= doc.text_field :doc_type %>
<% end %>
<%= f.submit %>

I've used fields_for and not f.fields_for.
I've seen that there's no need to use accept_nested_attributes.