public function load(ObjectManager $manager)
{
$entityClass = $this->getEntityClass();
$rootNode = new $entityClass();
$rootNode->setName('Root Node');
$firstCategory = new $entityClass();
$firstCategory->setParent($rootNode);
$firstCategory->setName('First Category');
$secondCategory = new $entityClass();
$secondCategory->setParent($firstCategory);
$secondCategory->setName('Second Category');
$manager->persist($rootNode);
$manager->persist($firstCategory);
$manager->persist($secondCategory);
$manager->flush();
$this->addReference($this->getReferencePrefix() . '.root', $rootNode);
$this->addReference($this->getReferencePrefix() . '.first', $firstCategory);
$this->addReference($this->getReferencePrefix() . '.second', $secondCategory);
}