Filecolumn in Edit View - restful form - Update existing Image

I generated code with ruby script/generate scaffold_resource

Whenever I go to edit an image in filecolumn it creates a whole new
record with the new image, as opposed to overwriting the image.

My Question is about the second block of code. How can I pass the ID
so that I can update the record instead of creating a duplicate with a
different image?

Here is my code.

Create View (works fine):
<h2>New kellymadison</h2>

<%= error_messages_for :km %>

<% form_for(:km, :url => kms_path,
                      :html => { :multipart => true }) do |f| -%>
  <p>
    <label for="Title">Title:</label>
    <%= f.text_field :title %>
  </p>
  <p>
    <label for="Description">Description:</label>
    <%= f.text_area :description %>
  </p>
  <p>
    <label for="wmv">URL for WMV:</label>
    <%= f.text_field :wmv_link %>
  </p>
    <p>
    <label for="wmv">URL for MOV:</label>
    <%= f.text_field :mov_link %>
  </p>
  <p>
    <%= file_column_field "km", "image" %>
  </p>
  <p>
    <%= submit_tag 'Create' %>
  </p>
<% end -%>

<%= link_to image_tag("/images/back.gif"), km_path, :class => 'back-
btn' %>

Edit View (The VIEW I AM HAVING TROUBLE WITH. It creates a new record
witha new ID)

<h2>Edit km</h2>

<%= error_messages_for :km %>
<% form_for(:km, :url => km_path,
                      :html => { :multipart => true }) do |f| -%>
   <p>
    <label for="Title">Title:</label>
    <%= f.text_field :title %>
  </p>
  <p>
    <label for="Description">Description:</label>
    <%= f.text_area :description %>
  </p>
  <p>
    <label for="wmv">URL for WMV:</label>
    <%= f.text_field :wmv_link %>
  </p>
    <p>
    <label for="wmv">URL for MOV:</label>
    <%= f.text_field :mov_link %>
  </p>
    <p>
     <% @km = Km.find(params[:id]) %>
    <%= file_column_field "km", "image" %>
  </p>
  <p>
    <%= submit_tag "Update" %>
  </p>
<% end %>

Please let me know if you find a solution.