How to insert a null value into DB using a radio_button with form_for

Hi,

I'm trying to add a third option for a filter using a radio button. It's
the old null-value-for-a-boolean story. Basically i'd like to add
another radio button labelled "No Filter" which would insert a null
value into the database when selected. Here is my first attempt. The
third button is showing up, but the null value is not being inserted.
What am I missing?

<div class="new-partner-form">
<%= form_for [:admin, matching_profile.partner, matching_profile],
:html => {:id => "edit_profile", :multipart => true} do |f| %>
  <div class="rounded-block semi-wide clear">
    <h4>Military Service</h4>
    <%= f.radio_button :served_in_us_army, false %>
    <%= label :served_in_us_army, 'NO', {:style => 'display:inline'} %>
    <%= f.radio_button :served_in_us_army, true %>
    <%= label :served_in_us_army, 'YES', {:style => 'display:inline'} %>
    <%= f.radio_button :served_in_us_army, nil %>
    <%= label :served_in_us_army, 'No Filter', {:style =>
'display:inline'} %>
    <%= f.error_message_on :served_in_us_army %>
  </div>

Hi,

I'm trying to add a third option for a filter using a radio button. It's
the old null-value-for-a-boolean story. Basically i'd like to add
another radio button labelled "No Filter" which would insert a null
value into the database when selected. Here is my first attempt. The
third button is showing up, but the null value is not being inserted.
What am I missing?

The parameters a rails app gets from a form are always strings,
because that's all a form can submit. You'll need to turn the value
back into a nil server-side.

Fred