buttons for a drop down list

The "show" for a row from feeds works correctly, showing both rows which
that feed row. However, I'd like to add a "add" button (and a "remove"?)
to the "Editing feed" page so that the the rows from categories will be
associated/unassociated with that feeds row.

(There's a many-to-many relationship between "feeds" and "categories".)

Feed: feed1
Edit | Back
Itemized Categories
cat1
category2

<http://localhost:3000/feeds/show/1>

Feed: feed2
Edit | Back

<http://localhost:3000/feeds/show/2>

Editing feed

Feed

Category
Show | Back

<http://localhost:3000/feeds/edit/1>

thufir@arrakis ~/Desktop/strawr $
thufir@arrakis ~/Desktop/strawr $ tail app/views/feeds/show.rhtml --
lines=13

<% if @feed.has_categories? %>
  <h3>Itemized Categories</h3>
  <table>
  <% for category in @feed.categories %>
    <tr>
      <td><%= category.category %></td>
    </tr>
  <% end %>
  </table>
<% end %>

thufir@arrakis ~/Desktop/strawr $
thufir@arrakis ~/Desktop/strawr $ tail app/views/feeds/_form.rhtml --
lines=2

<p><label for="category_id">Category</label><br/><%= select("post",
"category_id", Category.find_all.collect {|c| [c.category, c.id]}) %></p>
thufir@arrakis ~/Desktop/strawr $

thanks,

Thufir