File not assigned

photos.rb

#/model/user.rb
  attr_accessible :name, :first_name, :last_name, :email, :password, :password_confirmation, :remember_me,
                  :provider, :uid, :photos_attributes

  has_many :photos
  accepts_nested_attributes_for :photos
  

#/model/photo.rb
class Photo < ActiveRecord::Base
  attr_accessible :id, :photo, :user_id
  belongs_to :user
  mount_uploader :photo, PhotoUploader
end

#/controllers/users_controller.rb
def show
  @photos = @user.photos
end

def update
  @user.photos.build
end

#/views/devise/registration/edit.html.erb
<%= simple_form_for(resource, :as => resource_name, :url => registration_path(resource_name), :html => { :method => :put, :class => 'form-vertical', :multipart => true }) do |f| %>
  <%= f.error_notification %>
  
  ...

  <%= f.fields_for :photos do |p| %>
    <%= p.file_field :photo %>
  <% end %>
  
  ...

<% end %>