### Product model ###
class Product < ActiveRecord::Base
  has_one :line_item, :foreign_key => 'item_id', :conditions => {:kind => 2}
  validates_presence_of :kind, :description

  def price
    line_item.price
  end

  def line_item_update_attributes=(line_item_attributes)
    line_item_attributes.each do |attributes|
      line_item.update_attributes(attributes.last)
    end
  end

end

### View for product ###

<h2>Editing product</h2>

<%= error_messages_for :product %>

<% form_for(@product) do |f| %>
  <p>
    <b>Kind of Product</b><br />
    <%= f.text_field :kind %>
  </p>

  <p>
    <b>Description</b><br />
    <%= f.text_field :description %>
  </p>

  <p>
    <b>Price</b><br />
	<% fields_for "product[line_item_update_attributes][]", @product.line_item do |lif| %>
	    <%= lif.text_field :price %>
	<% end %>
  </p>

  <p>
    <%= f.submit "Update" %>
  </p>
<% end %>

<%= link_to 'Show', @product %> |
<%= link_to 'Back', products_path %>