Parameter being stripped form Search form with Restful Routes

I am having trouble with a search form and restful routes.

In my app I have a Topic model and a Classroom model. Topics can
exist in classrooms but don't have to so for that reason I have not
nested them.

I want to have a form that passes a topic_id and the search
parameter. So I have a form that looks like this:

<% form_tag ({:action => "show", :topic_id => @topic.id, :id =>
@classroom.id} , {:method => 'get'}) do %>
<p>
  <%= text_field_tag :search, params[:search] %>
  <%= submit_tag "Search", :name => nil %>
</p>
<% end %>

But when I submit the form the search param is posted but the topic_id
param is stripped. So when searching for 'gen' I get:

/classrooms/1?search=gen

instead of:

classrooms/1?search=gen&topic_id=43

The strange thing is that if I embed this in a link_to (setting the
additional param :search to a fixed value) the topic_id param doesn't
get stripped. It is only when it is submitted via the form that the
topic_id parameter is stripped.

Any ideas?

Thanks