Nested Forms - how to displayed the attributes content ?

I have a User model

class User < ActiveRecord::Base
has_one :account
  accepts_nested_attributes_for :account, :allow_destroy => true
  validates_associated :account
  attr_accessible :account_attributes

is working fine, validating and updating both records (User and
Account), but I caanot display the value in the form when is updated,
ex: below the firstname is not displayed but it's in the db account
#<User id: 1, email: "", ..
#<Account id: 4, user_id: 1, title: 0, first_name: "Yves", ...

my view
    <% form_for :user, :url => { :action => "update" } do |user_form|
      <% user_form.fields_for :account_attributes do |account_fields| %>

        <label for="user_firstname">First Name</label>
        <%= account_fields.text_field :first_name, :size => "30" %>

what am I missing ?

thanks for your help


Discover the issue : I had to write correctly my form_for

  <% form_for :user, @user, ...

and not <% form_for :user