Which parameters should I pass to form_set_value()?

Question

I am trying to update a value during the validate phase of a node form: When the custom validation error is fired, I empty one of the fields.

I am trying to use the following code.

function eventcheck_node_validate($node, $form, &$form_state) {
  if ($node->type == 'article') {
    if ((array_key_exists('endpoints',$node->field_event_choose['und'][0])) && (($node->field_event_title['und'][0]['value']<>'') || (isset($node->field_event['und'][0]['value'])))) {
      $form_set_value(???);
      $form_set_error('event_checker_group',t('Error message'));
    }
  }
}

I understand that the last parameter for form_set_value() is the same $form_state the validation handler gets. What are the $element and the $value?

I was trying a lot and apparently the desired value resides in $form['field_name']['und'][0]['value']['#value'] and only there. When I try form_set_value($form['field_name']['und'][0]['value']['#value'],'foo',$form_state), I get this error.

Recoverable fatal error: Argument 2 passed to drupal_array_set_nested_value() must be an array, string given, called in /includes/form.inc on line 2436 and defined in drupal_array_set_nested_value()

I also tried the following code.

$newvalue = $form['field_name']['und'][0]['value'];
$newvalue['#value']='foo';
form_set_value($form['field_name']['und'][0]['value'],$newvalue,$form_state);

I get this error.

Warning: mb_strlen() expects parameter 1 to be string, array given in drupal_strlen()

Answer 1

After a lot of debugging, I finally managed to make this work. The trick lies inside $form['complete form']. but first things first, how does form_set_value() work and what does it do?

The form_set_value() function

As the documentation suggests:

If you want to update the value of $form['elem1']['elem2'], which should be stored in $form_state['values']['elem1']['elem2'], you would set $element['#parents'] = array('elem1','elem2').

Now what does that mean? In my case, I had a textfield called 'field_event_title', which is the name I gave it on creation. In $form, all fields have a sub-array in $form['field_name'], which in my case is $form['field_event_title']. This is where the submitted value also is stored. Since it is a textfield, Drupal maintains both the language and the delta [question for editors: is this right?] of the input data. So in fact, the value is not stored in $form['field_name']['value'], but in $form['field_name']['und'][0]['value'] (['und']=language; [0]=delta). Note that "und" is the Drupal key for the default language of the site; if it is in German, then it would be "de"; however, in most cases it should be "und."

To actually change the value using form_set_value(), one is ought to invoke the function by writing:

form_set_value($form['field_name'], array('und' => array(0 => array('value' => 'foo'))), $form_state);

Updating a form to repopulate it with different values than submitted (or clearing them)

Thus far, this was what Krister Andersson suggested, but that did not work in my case, since I wanted to clear fields once a custom validation error has been invoked. One would suspect that the form repopulates itself using the values inside $form_state['values'] (which is actually the place where the values are stored, the actual place that gets updated when using form_set_value() and the place which generates the $form later), but that is not the case: It uses the values inside $form_state['complete form'], which is a 'copy' of $form (notice that it is spelled 'complete form', with a space, not an underscore).

Using $form_state['complete form']['field_name']['und'][0]['value']['#value'] = 'foo'; is what updates the values that actually repopulate the form on a validation error. (Note: you can, as do I in my usecase, set it to =NULL to simply empty the field.)

Summary

Where is the difference between $form['field_name'] (e.g. updating through form_set_value()) and $form['complete form']? Well, the former updates the actual value, which then gets stored inside the database; the latter is being used to repopulate a form when it failed a validation.

Answer 2

I managed to get it working by doing this:

// hook_form_alter, check so it's the correct form and attach a validate function
function sample_form_alter(&$form,&$form_state,$form_id) {
  if ($form_id == "sample_dummy_node_form") {
    $form['#submit']['#validate'] = 'sample_dummy_node_form_validate';
  }
}

// validate function, wich just sets the `title` field to `tjoohooo` using `form_set_value`
function sample_dummy_node_form_validate($form, &$form_state) {
  $form['title']['#parents'] = array('title');
  form_set_value($form['title'],t('tjoohooo'), $form_state);
}

If you haven't done so already, you should have a look at the documentation form_set_value

* EDIT *

In you example I guess the code in your validate method should be:

function hook_node_validate($node,$form,&$form_state) {
  if (fancy_stuff){
    $form['field_name']['#parents'] = array('field_name');
    form_set_value($form['field_name'],t('foo'),$form_state);
  } 
}

From the documentation for the parameters:

$element: The form element that should have its value updated; in most cases you can just pass in the element from the $form array, although the only component that is actually used is '#parents'. If constructing yourself, set $element['#parents'] to be an array giving the path through the form array's keys to the element whose value you want to update. For instance, if you want to update the value of $form['elem1']['elem2'], which should be stored in $form_state['values']['elem1']['elem2'], you would set $element['#parents'] = array('elem1','elem2').