I'm using Drupal 8 and I have defined a custom entity type 'my_entity_type', with form handlers defined in the @ContentEntityType annotation:

 * @ContentEntityType(
 *   handlers = {
 *     "form" = {
 *       "add" = "Drupal\my_module\Form\MyEntityForm",
 *       "edit" = "Drupal\my_module\Form\MyEntityForm",
 *       "delete" = "Drupal\my_module\Form\MyEntityDeleteForm",


  • MyEntityForm extends ContentEntityForm
  • MyEntityDeleteForm extends ContentEntityConfirmFormBase

These are routed in my_module.routing.yml leveraging the _entity_form element:

_entity_form: my_entity_type.add

I am rendering the form from another form as a modal:

$form['add_my_entity_button'] = [
  '#type' => 'link',
  '#title' => t('Add NewEntity'),
  '#url' => Url::fromRoute(
      'node' => is_numeric($node) ? $node : NULL,
  '#attributes' => [
    'class' => ['button', 'use-ajax'],
    'data-dialog-type' => 'modal',
  '#attached' => [
    'library' => ['core/drupal.dialog.ajax'],

All works as imagined. The '#required' type validator appears in the form in the modal. Modal Validation Working


To add custom validation to this form's elements ('live' validation that does not close the modal). I recognize that this type of problem isn't novel, and I've done it often in non-modal forms. The methods I would usually leverage are:

  • '#element_validate' form element callback
  • validateForm() type form callback

Both appear to require submission of the form, close the modal, and render the errors in a flat form display. I then combed through Core for ideas and embarked on a remarkably twisted yak-shaving journey:

  1. Adding validation via BaseFieldDefinition::addConstraint in the entity baseFieldDefinitions() : This still submitted the form and led to a flat (non modal) form with errors correctly thrown. Once I solve this problem, I will likely leave this in to serve as a gatekeeper at the storage level.

  2. Extending the 'number' FieldType via Plugin, extending the widget, type or storage : This seemed overly complex for something as simple as adding a validator. I bailed on this before getting too far, since I considered:

  3. Injecting a generic '#ajax' 'replace' callback to the element inside buildForm() in MyEntityForm : I could NOT get ajax inserted in this manner to fire here.

  4. Injecting a generic '#ajax' 'replace' callback to the element in hook_form_alter : I could NOT get ajax callbacks inserted in this manner to fire either.


What have I missed? I feel this is simple. Is there something I've missed in Form that defines simple ajax validation?

Is there a standard way for validating form elements inside modals without closing the modal?

Can AJAX 'replace' callbacks be attached to forms defined and routed via the @ContentEntityType annotation? Is there a restriction here in the render pipeline?

1 Answer 1


This is a excellent article which you can read for "fixing" your issue: http://www.mediacurrent.com/blog/loading-and-rendering-modal-forms-drupal-8

Keep in mind this lines:

First, we wrap our form with a div

public function buildForm(array $form, FormStateInterface $form_state, $options = NULL) {
  // It is important this id
  $form['#prefix'] = '<div id="modal_example_form">';
  $form['#suffix'] = '</div>';

  // The status messages that will contain any form errors.
  $form['status_messages'] = [
    '#type' => 'status_messages',
    '#weight' => -10,

  // more code

  $form['actions']['send'] = [
      '#type' => 'submit',
      '#value' => $this->t('Submit modal form'),
      '#attributes' => [
        'class' => [
      // Function to call when the users click on submit button.
      '#ajax' => [
        'callback' => [$this, 'submitModalFormAjax'],
        'event' => 'click',

    // more code

Second, we show the errors:

 * AJAX callback handler that displays any errors or a success message.
public function submitModalFormAjax(array $form, FormStateInterface $form_state) {
  $response = new AjaxResponse();

  // If there are any form errors, re-display the form.
  if ($form_state->hasAnyErrors()) {
    // Remember the previous id ? Here it is
    $response->addCommand(new ReplaceCommand('#modal_example_form', $form));
  else {
    $response->addCommand(new OpenModalDialogCommand("Success!", 'The modal form has been submitted.', ['width' => 800]));

  return $response;

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