Drupal 7 - node form - dsm/dpm does not work with own validate/submit function

Question

I want to add an own validation (or submit) handler/function to my existing node form. I tried multiply things (following), but it does not work properly.

function MY_MODULE_form_alter(&$form, &$form_state, $form_id) { 
  $form['#validate'][] = 'MY_MODULE_handler';
//or
  $form['actions']['submit']['#validate'][] = 'MY_MODULE_handler';
//or
  $form['submit']['#submit'][] = 'MY_MODULE_handler';
//or
  $form['#submit'][] = 'MY_MODULE_handler';
//or
  array_unshift($form['#validate'], 'MY_MODULE_handler');
//or
  $form['actions']['submit']['#submit'][]='MY_MODULE_handler';
}

But none of these attempts seem to work with my handler:

function MY_MODULE_handler($form, &$form_state){
  dsm($form);  
  dsm($form_state);
}

Looking at the $form variable in form_alter I found that $form['actions']['submit']['#submit'] and $form['#validate'] have already a submit/validate handler.

But if I use one of the examples above, nothing is printed by dsm. In addition it does not print the confirmation (of submitting) anymore! But if I use

function MY_MODULE_handler($form, &$form_state){
  die('Handler was hit');
}

I see that the handler was hit.

What can be the reason? I need the handler to read two form fields and save them in one node attribute.

Answer 1

As @2pha proposed, I used instead of an additional validation/submit handler the hook HOOK_node_presave which allowed me to solve my problem (which was to assign values from the form to one node attribute, see comments).