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| %>

<%= option.food_item.food_name %>
<%= 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