Multistep Nested Forms Using Rails

Question

I'm trying to implement a multistep form using a nested form, but since I'm using a nested form, it's not working out like the railscast. I got this from railscast #217 (multistep form/wizards) http://railscasts.com/episodes/217-multistep-forms

old _event.html.erb form

<%= render "event_details", :f => f %>

<%= render "sandwich_details", :f => f %>

new version of _event.html.erb

<%= render "#{@event.current_step}_details", :f => f %>

I get a "undefined method current_step for nil:NilClass" error. I think this is because event is nested inside user, so I'm not directly using a forms_for method like in the railscast. Here's the loop leading up to this point. I followed the railscast up to the point where he refreshed the page for the first time.

_forms.html.erb

<% form_for @user do |f| %> 
.
.
.
   <% f.fields_for :events do |event_form| %>
      <%= render :partial => 'event', :locals => {:f => event_form } %>
   <% end %> 
<% end %>

event.rb

class Event < ActiveRecord::Base
  belongs_to :user
  attr_writer :current_step

  def current_step
    @current_step || steps.first
  end

  def steps
    %w[event_details item_details]
  end

How can I change "#{@event.current_step}_details" to make it find current_step?

EDIT

It turns out I had to pass @event => :event into the locals, and then add @event = Event.new to the base model new method in the controller.