Radio button with Observe field

hi,
i want a observe field to get which radio button has selected , i was
tried , but i was returning both ids when clicked on one among two
My View code looks like this:
<% fm.food_item.food_options.each do |option| %>
<div class="nutrition_food_option">
<input type="radio" id="<%= option.food_item.id %>"
name="sub_food_taken" value="yes" />
<label for="<%= fm.id %>"><%= option.food_item.food_name %></label>
</div>
<%= observe_field("option.food_item.id",
    :frequency => 0.5,
    :url => { :action => :modify_subfood_intake, :fm =>
option.food_item.id, :date => date},
    :with => "'sub_food_taken='+value") %>
<% end %>
is it i should call observe field out side loop, then i dont get
food_item.id !!

any ideas
thanks