Modifying your forms in drupal

Drupal forms can be modified by the form alter hook. This performs alterations to your form before it is rendered. It is especially useful when you are writing a module which modifies another module. For example, a mypage.module could modify the page module by removing the comment and author fieldset along with the log message textarea and adding two new textfields.

First time users of hook_form_alter often get frustrated since their modifications work on certain fields and not others. This is due to the fact that drupal loads modules in alphabetical order and overriding modules, such as mypage.module mentioned above, must be loaded after the modules they override. There are two ways to solve this problem. You can change name of your module such that is loaded after the module you wish to override. A better way would be to go into the system table is the database and modify the weight of the module. All modules are assigned the weight zero by default. Therefore, modifying the weight of your module form 0 to 1 would assure that your module would be loaded in the desired order.

Modifying a form is a form step process:

Step 1: Define a _form_alter form

function mypage_form_alter($form_id, &$form) {

  return $form
}

The function name must end with _form_alter. The function takes $form_id and a reference to the $form array as parameters. This function returns the $form array reference after modifications.

Step 2: Define a _form_alter form

function mypage_form_alter($form_id, &$form) {
  if($form_id == 'mymodule_node_form') {

  }
  return $form
}

It is prudent to test for your form_id. This would serve to eliminate a number of difficult to debug errors. If you do not the name of your form, view the form source in the browser. It should look something like:

<input type="hidden" name="edit[form_id]" id="edit-mypage-node-form" value="mypage_node_form" />

Thus the form_id is mymodule_node_form.

Step 3: Modify or unset undesirable fields

function mypage_form_alter($form_id, &$form) {
  if($form_id == 'mymodule_node_form') {
    // modify attributes of existing fields
    $form['author'] = array('#type' => 'hidden');
    $form['options'] = array('#type' => 'hidden');

    // remove form fields
    unset($form['menu']);
    unset($form['path']);
  }
  return $form
}

In this example, I set two fieldsets to hidden. This has the same effect as removing a field from the user's point of view. However, the difference is significant from a module developer's point of view. Hidden fields allow a developer to pass defined values. unset() simply removes the form elements from the $form array.

Step 4: Adding new fields

function mypage_form(&$node) {
  $form['field1'] = array(
    '#type' => 'textfield', 
    '#title' => t('field1'), 
    '#required' => TRUE, 
    '#default_value' => $node->field1, 
    '#weight' => -5
  );
  $form['field2'] = array(
    '#type' => 'field2', 
    '#title' => t('field2'), 
    '#default_value' => $node->field2,
    '#required' => TRUE
  );
}

As you can see, adding fields to a form simply involves adding fields as you would add to a any drupal form. You just have to make sure that you add your field to the right form array. Page module uses node module forms. Therefore, we add to the $node array.

Beware

Node form fields post data in database tables. When you unset a field, a default value is posted in its place. For example, if you unset author, the uid field in node table would get a 0 value by default.