Adding Constraint to entity referenced Paragraph field

Question

I'm attempting to follow this documentation to add a custom constraint to a field in a Paragraph but I'm having trouble in adding constraint to that field.

Currently I have book content type with paragraph type bibliographic_information. This paragraph type has a field named as field_book_edition. This field accepts a string value. I'd like to be able to add a constraint to the field so that it accepts a certain string pattern as input.

I've created constrain and tried using hook_entity_bundle_field_info_alter, when ever I try to save the node I am getting the following error.

The website encountered an unexpected error. Please try again later.

InvalidArgumentException: Field field_bibliographic_information is unknown. in Drupal\Core\Entity\ContentEntityBase->getTranslatedField() (line 587 of core/lib/Drupal/Core/Entity/ContentEntityBase.php).

My Code is as follows :

book_content.module

use Drupal\Core\Entity\EntityTypeInterface;

/**
 * Implements hook_entity_bundle_field_info_alter().
 */
function book_content_entity_bundle_field_info_alter(&$fields, EntityTypeInterface $entity_type, $bundle) {

  // Refer https://drupal.stackexchange.com/a/277438/

  if ($entity_type->id() === 'paragraph' && $bundle === 'bibliographic_information' && isset($fields['field_book_edition'])) {

    // Add our custom validation to the book edition field.
    $fields['field_book_edition']->addConstraint('BookContentEditionConstraint');
  }

}

BookContentEditionConstraintValidator.php

namespace Drupal\book_content\Plugin\Validation\Constraint;

use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;

/**
 * Validates the Book Content constraint.
 */
class BookContentEditionConstraintValidator extends ConstraintValidator {

  /**
   * {@inheritdoc}
   */
  public function validate($entity, Constraint $constraint) {

    $node = $entity->getEntity();

    // Get referenced paragraph entities.
    $referenceItem = $node->get('field_bibliographic_information')->referencedEntities();

    foreach ($referenceItem as $data) {

      $fields = [
        'field_book_author' => $data->field_book_price->value,
        'field_book_edition'  => $data->field_book_edition->value,
        'field_book_language'  => $data->field_book_language->value,
        'field_book_pages'  => $data->field_book_pages->value,
        'field_book_price'  => $data->field_book_price->value,
        'field_book_published_year'  => $data->field_book_published_year->value,
        'field_book_synopsis'  => $data->field_book_synopsis->value,
        'field_website'  => $data->field_website->getValue()[0]['uri'],
      ];
    }

    // Check if the book edition matches or not.
    if ($fields['field_book_edition'] != 'asdfg') {

      $this->context->addViolation($constraint->invalidEdition, ['@edition' => $fields['field_book_edition']]);
    }

  }

}

Attached the Node and Paragraph structures screenshot.

I have gone through similar questions and read their comments/answers but coundn't find what exactly is the error in the code.

Add Constraint to Paragraph Field answer by Carlos

Adding Constraint to Paragraph field comment by 4k4

I implemented the same what is mentioned in both the links.But still it is not working.

For debugging purpose I have added the above constraint to a node field and it worked as expected.

From what I read if you provide the correct $entity_type->id() all the fields for the particular entity id will be rendered in $fields variable in hook_entity_bundle_field_info_alter , is my understanding is correct ?

Should I be implementing hook_entity_bundle_field_info_alter or hook_form_alter for adding a custom constraint to a paragraph field ?

EDIT 1:

After further debugging if I add

if ($node->hasField('field_bibliographic_information')) {
   \Drupal::logger('book_content')->info('Hello');
 $referenceItem = $node->get('field_bibliographic_information')->referencedEntities();
 ....remaining code.....
 }

in validate method the code inside the loop doesn't get executed.

Answer 1

I am able to achieve this finally.

Basically what I was doing wrong was I was assuming the $entity is a node object and but it actually is paragraph object.

<?php

namespace Drupal\book_content\Plugin\Validation\Constraint;

use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;


/**
 * Validates the Book Content constraint.
 */
class BookContentEditionConstraintValidator extends ConstraintValidator {


  /**
   * {@inheritdoc}
   */
  public function validate($items, Constraint $constraint) {

    if (!$item = $items->first()) {
      return;
    }

    $field_name = $items->getFieldDefinition()->getName();

    /** @var \Drupal\Core\Entity\EntityInterface $entity */
    $entity = $items->getEntity();

    if ($entity->hasField('field_book_edition')) {

      $edition = $entity->field_book_edition->value;


      // Check if the book edition matches or not.
      if ($edition != 'asdfg') {

        $this->context->addViolation($constraint->invalidEdition, ['@edition' => $edition]);
      }

    }

  }

}