How do I programmatically assign a new book when a new node is created?

Question

For a given content type, if a new node is created and no book is assigned, I'd like to create a new book and assign the new node to that book.

(some background - that content type is the book cover, and for editors to create a new book I ask them to create a cover first. this makes more sense for them. The 'add child page' links will then appear automatically. I know they can select 'new book' on the create page, but that makes no sense in that context - alternatively, if I could have that selected by default, that would work )

Answer 1

you can use a hook implementation in a custom module

    /**
 * Implements hook_node_insert().
 */
function mymodule_node_insert($node) {
  // Check if the node is of the desired content type and no book is assigned.
  if ($node->getType() == 'my_content_type' && empty($node->book['bid'])) {
  // Get the book storage handler.
  $bookStorage = \Drupal::entityTypeManager()->getStorage('book');

  // Create a new book entity.
  $book = $bookStorage->create(array(
    'type' => 'book',
    'title' => $node->getTitle(),
    'uid' => $node->getOwnerId(),
  ));
    $book->save();

    // Assign the node to the new book.
    $node->book['bid'] = $book->id();
    $node->book['plid'] = 0;
    $node->save();
  }
}

Answer 2

Using @rafael 's answer:

/**
 * Implements hook_node_insert().
 *
 * - assigns a new book for a new MYCONTENTTYPE if none was selected
 */

function MYMODULE_node_insert($node) {
  if ($node->getType() == 'MYCONTENTTYPE' 
    && property_exists($node,'book') 
    && empty($node->book['bid'])) {
      $node->book['bid'] = 'new';
      $node->book['plid'] = 0;
      $node->save();
  }
}