Adding Ajax on existing dropdown list I added this code in my template.php

function PORTO_SUB_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'rapporto_di_lavoro_node_form') {

    $form['field_livello']['und']['#ajax'] = array(
                                  'callback' =&gt; 'my_test_callback',
                                  'wrapper' =&gt; 'callback_wrapper',
                                  'progress' =&gt; array(
                                    'message' =&gt; 'caricamento',
                                    'type' =&gt; 'throbber',
                                    ),
                                );        }}

function my_test_callback($form, &amp;$form_state) {
dpm($form_state); 
}


When I select an item I can see the throbble, but no dpm() message.
What's wrong?

As I'm still not modifying fields, I don't need to unset or indicate wrapper, aren’t I?
similar question is here.

Dynamic select list in the form (dependent dropdown)

you can get your solution from here.