gistfile1.txt

# routes.rb

match "/admin/by_mark/:id" => "admin/models#by_mark", :as => "by_mark"

# view

  <script>
    $(function(){
      $("#mark").change(function(){
        load_model($(this).val());
      });

      function load_model(mark) {
        $.getJSON("<%= by_mark_path(mark) %>", {ajax: 'true'}, function(j) {
          var options = '<option value="">Selecione o Modelo</option>';
          for (var i = 0; i < j.length; i++) {
            options += '<option value="' + j[i].model.id + '" '+selected+'>' + j[i].model.name + '</option>';
        }
        $("#model_id").html(options);
      });
    });
  </script>


collection_select(:state, :id, State.find(:all), value, :name, {:include_blank => "Selecione o Estado", :selected => selected}, {:name => name})