How to add extra validate hook to the order information form?

Question

I am trying to collect additional information on checkout to add to the order. I have added new fields to the order entity and then used hook_form_commerce_checkout_flow_multistep_default_alter to add "fake" fields to the order_information form. Then, using hook_entity_presave(), I take the values from the fake fields and set the real order fields. This all works as expected.

As some of these fake fields are conditionally required to be entered, I am trying to add a validate function. In the form alter I have added $form['actions']['next']['#submit'][] = '_altpay_form_validate'; and for that function I have:

function _altpay_form_validate(&$form, $form_state) {
  $who_paying = $form_state->getValue('altpay');
  if (!$who_paying) {
    $form_state->setErrorByName('altpay', t('You must select who is paying for this order.'));
  }
}

Calling setErrorByName() crashes with the following exception.

LogicException: Form errors cannot be set after form validation has finished.

I tried setting Validation Complete to FALSE and then setErrorByName() no longer crashes, but also does not prevent form submission.

I suspect this has something to do with the multi-step nature of this form.

Do you have any hint on what I am missing?

Answer 1

$form['actions']['next']['#submit'][] = '_altpay_form_validate';

You're actually adding it to the submit function. Which makes sense the errors that you're seeing since it's too late to do any form validation here.

Needs to be:

$form['actions']['next']['#validate'][] = '_altpay_form_validate';