class EntityExists

class EntityExists extends ProcessPluginBase implements ContainerFactoryPluginInterface {

  /**
   * The entity storage.
   *
   * @var \Drupal\Core\Entity\EntityStorageInterface
   */
  protected $storage;

  /**
   * EntityExists constructor.
   *
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin ID.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param $storage
   *   The entity storage.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, EntityStorageInterface $storage) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->storage = $storage;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration = NULL) {
    return new static($configuration, $plugin_id, $plugin_definition, $container
      ->get('entity_type.manager')
      ->getStorage($configuration['entity_type']));
  }

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    if (is_array($value)) {
      $value = reset($value);
    }
    $entity = $this->storage
      ->load($value);
    if ($entity instanceof EntityInterface) {
      return $entity
        ->id();
    }
    return FALSE;
  }

}