loadBookLink example

elseif ($node->book['pid'] < 0) {
      // -1 is the default value in BookManager::addParentSelectFormElements().       // The node save should have set the bid equal to the node ID, but       // handle it here if it did not.       $node->book['pid'] = $node->book['bid'];
    }

    // Prevent changes to the book outline if the node being saved is not the     // default revision.     $updated = FALSE;
    if (!$new) {
      $original = $this->loadBookLink($node->id(), FALSE);
      if ($node->book['bid'] != $original['bid'] || $node->book['pid'] != $original['pid'] || $node->book['weight'] != $original['weight']) {
        $updated = TRUE;
      }
    }
    if (($new || $updated) && !$node->isDefaultRevision()) {
      return FALSE;
    }

    return $this->saveBookLink($node->book, $new);
  }

  

if (isset($user_input['table'])) {
      $order = array_flip(array_keys($user_input['table']));
      $form['table'] = array_merge($order, $form['table']);

      foreach (Element::children($form['table']) as $key) {
        if ($form['table'][$key]['#item']) {
          $row = $form['table'][$key];
          $values = $form_state->getValue(['table', $key]);

          // Update menu item if moved.           if ($row['parent']['pid']['#default_value'] != $values['pid'] || $row['weight']['#default_value'] != $values['weight']) {
            $link = $this->bookManager->loadBookLink($values['nid'], FALSE);
            $link['weight'] = $values['weight'];
            $link['pid'] = $values['pid'];
            $this->bookManager->saveBookLink($link, FALSE);
          }

          // Update the title if changed.           if ($row['title']['#default_value'] != $values['title']) {
            $node = $this->nodeStorage->load($values['nid']);
            $node = $this->entityRepository->getTranslationFromContext($node);
            $node->revision_log = $this->t('Title changed from %original to %current.', ['%original' => $node->label(), '%current' => $values['title']]);
            $node->title = $values['title'];
            

return new static(
      $container->get('book.manager')
    );
  }

  /** * {@inheritdoc} */
  public function validate($entity, Constraint $constraint) {
    if (isset($entity) && !$entity->isNew() && !$entity->isDefaultRevision()) {
      /** @var \Drupal\Core\Entity\ContentEntityInterface $original */
      $original = $this->bookManager->loadBookLink($entity->id(), FALSE) ?: [
        'bid' => 0,
        'weight' => 0,
      ];
      if (empty($original['pid'])) {
        $original['pid'] = -1;
      }

      if ($entity->book['bid'] != $original['bid']) {
        $this->context->buildViolation($constraint->message)
          ->atPath('book.bid')
          ->setInvalidValue($entity)