Rails 3: Nested form_for child field_for parent

cities_controller.rb

  #Description: We want to show a region(parent) form
  #while showing City information.
  #If the form submitted with Region information, 
  #  - New region will be created
  #  - City#region_id will have the new region id


  def show
    @city = City.find(params[:id])
    @region = @city.build_region

    respond_to do |format|
      format.html # show.html.erb
      format.json { render json: @city }
    end
  end


  #Note: This is just the default method provided by a scaffold. 
  #Nothing special needed to handle this.
  def update
    @city = City.find(params[:id])

    respond_to do |format|
      if @city.update_attributes(params[:city])
        format.html { redirect_to @city, notice: 'City was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: "edit" }
        format.json { render json: @city.errors, status: :unprocessable_entity }
      end
    end
  end

models.rb

  #Models/City.rb
  class City < AciveRecord::Base
    belongs_to :region
    accepts_nested_attributes_for :region    #<---- This line will do the trick in cities_controller#update method
  end

  #Models/Region.rb
  class Region < AciveRecord::Base
    has_many :cities
  end
  

routes.rb

  #Defining the regions resource
  resources :regions do

      #This part doesn't really have any effect in this case
      #But in our example application, city is a nested resource
      #region
      resources :cities
  end

  #City is also a independent resource.
  #This way we can access /cities/1
  resources :cities

  #NOTE: Run `rake:routes` to see the output

show.html.erb

  <!-- 
     This is the form to enter new Region information.     
  -->

  <%= form_for(@city) do |f|%>
	<%= f.fields_for(:region) do |fr|%>
		<%= fr.text_field :name%>
	<% end %>
	<%= f.submit 'Submit'%>
  <% end %>