This CodeLet will ajaxify the respective element and will trigger an event. Respective action will be taken by the code and displayed on the form within the specified wrapper

Drupal Ajax form Event
CodeLet
<?php
/**
 * Implementation of hook_form_alter
 *
 * @param unknown $form
 * @param unknown $form_state
 * @param unknown $form_id
* @author DrupalD http://twtitter.com/DrupalD
 */
function mymodule_form_alter(&$form, &$form_state, $form_id) {
  switch(
$form_id) {
      case
'incident_form_node_form':
       
$form['client_er'] = array(
           
'#type' => 'item',
           
'#title' => t('Client\'s Email Recipients'),
           
'#description' => t('Once you select a client, you will see the email recipient for that client listed here'),
           
'#weight' => -49,
        );
       
       
$form['field_client']['und']['#ajax'] = array(
         
'callback' => '_mymodule_recipitn_js',
         
'wrapper' => 'edit-client-er',
        );
  }
}

/**
 * _mymodule_recipitn_js
 *
 * Return email recipint
 * @author DrupalD http://twtitter.com/DrupalD
 */
function _mymodule_recipitn_js($form, $form_state){
 
$am__recipient = array();
 
 
//Get the selected client
 
$sn__client_nid = $form_state['triggering_element']['#value'];
 
 
//Find all the email recipints for that client
 
$am__recipient = _get_node_email_recipient($sn__client_nid);

  if (!empty(
$am__recipient)) {
    
$form['client_er'] = array(
       
'#type' => 'item',
       
'#markup' => implode('<br />', $am__recipient),
       
'#title' => t('Client\'s Email Recipients'),
       
'#description' => t('Here are all the email recipients from selected client.
            If you do not add any new email recipints in #email_recipients field below, above details will be used for this incident'
, array('#email_recipients' => t('Email Recipients'))),
       
'#prefix' => '<div id="edit-client-er">'
       
'#suffix' => '<div id="edit-client-er">'
    );

    return  array(
      
'#type' => 'ajax',
      
'#commands' => array(
        
ajax_command_replace("#edit-client-er", render($form['client_er'])),
       ), 
    );
  }
  else {
    return
t('No email recipients found for this client');
  }
}


/**
 * _get_node_email_recipient
 *
 * @param unknown $sn__client_nid
 * @author DrupalD http://twtitter.com/DrupalD
 */
function _get_node_email_recipient($sn__client_nid) {
 
$result = db_select('field_data_field_email_recipients', 'fer')
            ->
fields('fer', array('field_email_recipients_email'))
            ->
condition('entity_id', $sn__client_nid, '=')
            ->
execute();
  while (
$am__email_recipient = $result->fetchAssoc()) {
   
$am__recipient[] = $am__email_recipient['field_email_recipients_email'];
  }
  return
$am__recipient;
}
?>
Submitted by DrupalD on