gistfile1.rhtml

<tr id='<%= item.html_id %>'>
  <td><%= link_to_remote "X", :url => cart_line_item_path(item), :confirm => "Are you sure you want to remove the item?", :method => :delete, :class => "remove"  -%></td>
  <td>
    <%= product_mini_image(item.product)%>
  </td>
  <td>
    <%= link_to_product_name(item.product, true)%>
    <% if item.size and item.product.valid_sizes %>
    <p>
      Size: <%= select_tag "size_select_#{item.id}", options_for_select(item.product.valid_sizes.collect{ |c| [c, c.to_s]}, item.size.to_s), {:onchange => remote_function(:url => { :controller => 'line_items', :action => 'size_select', :id => item.id }, :with => "'size=' + $('#size_select_#{item.id}').val()")}
       %>
    </p>
    <% end %>
    <% unless item.color.blank? %>
    <p>
      Color: <%= select_tag "color_select_#{item.id}", options_for_select(item.product.colors.collect{ |c| [c, c]}, item.color), {:onchange => remote_function(:url => { :controller => 'line_items', :action => 'color_select', :id => item.id }, :with => "'color=' + $('#color_select_#{item.id}').val()")} %>
    </p>
    <% end %>
  </td>
  <td class='price'>
    <%=number_to_currency(item.price, :precision => 0)-%>
  </td>
</tr>

Line Item Controller.rb

  def size_select
    change_attribute(:size)
  end
  
  def color_select
    change_attribute(:color)
  end
  
  def change_attribute(attribute)
    line_item = LineItem.find(params[:id])
    line_item.update_attribute(attribute, params[attribute])
    respond_to do |format|
      format.js { render :text => 'success'}
    end
  end