Context

EK application has a module that store personal documents for user. When user account is deleted, those documents may be transferred to another account.

To achieve that, we need to alter the user account cancel form when building the form, validating and submitting it.

Let's review the 3 steps.

BUILD

The form before altering it looks like this

We need to add a field to select another user account to which the document of the canceled account will be moved to.

To achieve that we Implements hook_form_alter() in MyModule.module:

function MyModule_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {    
  if ($form_id == 'user_multiple_cancel_confirm') {
        $form['move_uid_documents'] = [
      '#type' => 'textfield',
      '#title' => t('Move uer documents'),
      '#autocomplete_route_name' => 'MyModule.user_autocomplete',
      '#description' => t('Select to whom to transfer personal documents'),
    ];
    $form['#validate'][] = 'MyModule_form_user_delete_validate';
    $form['#submit'][] = 'MyModule_form_user_delete_submit';
    
    return $form;
      
  }
}

What we can notice here is:

• We alter selected form defined by form ID. In this case : "user_multiple_cancel_confirm";
• We create the required field by returning $form['move_uid_documents'] ;
• We add 2 new actions for validation, $form['#validate'][], and submit, $form['#submit'][],  for the next steps.

After altering the form will look like this:

We have a new field to select user. In our case, we also have an autocomplete function that helps selecting existing user. However, we need to ensure that the value entered in the field is really an existing user. This is the part handled by the validation.

VALIDATE

The validation is defined in MyModule_form_alter by adding validate callback named MyModule_form_user_delete_validate. Therefore, we need to create the function with thah particular name in MyModule.module.

function MyModule_form_user_delete_validate(&$form, \Drupal\Core\Form\FormStateInterface $form_state) {   
  if ($form['#form_id'] == 'user_multiple_cancel_confirm') {
        if ($form_state->getValue('move_uid_documents') <> '') {
            
            $query = "SELECT uid FROM {users_field_data} WHERE name = :n";
            $data = db_query($query, [':n' => $form_state->getValue('move_uid_documents')])
                    ->fetchField();
            if ($data) {
                $form_state->setValue('move_uid_documents', $data);
            } else {
                $form_state->setErrorByName('move_uid_documents', t('Unknown user to move documents'));
            }
            
 
        }
    
     return $form;
      
  }

Here the function will check against user_field_data table that the id is valid.

If not an error message will be displayed:

However, if valid, we store the value to be used in the next step which is the submission.

SUBMISSION

As for validation, the submission is defined in MyModule_form_alter by adding validate callback named MyModule_form_user_delete_submit.

function MyModule_form_user_delete_submit(&$form, \Drupal\Core\Form\FormStateInterface $form_state) {
    
  if ($form['#form_id'] == 'user_multiple_cancel_confirm') {
    if($form_state->getValue('move_uid_documents')){
        foreach($form_state->getValue('accounts') as $key => $id) {               
        \Drupal::database()->update('MyModule_table')
           ->fields(['uid' => $form_state->getValue('move_uid_documents'), 'folder' => t('Moved from user @u', ['@u' => $id])])
           ->condition('uid', $id)->execute();
       }
    }
     \Drupal::messenger()->addStatus(t('Documents moved to user @u', ['@u' => $form_state->getValue('move_uid_documents')]));
     return $form;
      
  }
}

In the function above, we pick the id of each user account that is canceled and change to new user id in the document table.

The function also display a message to confirm actions: both cancellation and the submit hook have been executed.

Please feel free to comment or suggest improvements.

Thank you.