Document.php

<?php

namespace Application;

use Doctrine\ORM\Mapping as ORM;

/**
* @ORM\Entity
* @ORM\Table(name="documents")
* @ORM\HasLifecycleCallbacks
*/
class Document
{
  /**
   * @ORM\PrePersist
   */
  public function onPrePersistSetParent()
  {
    $repository = $this->getEntityManager()->getRepository(static::class);
    $parent = $repository->findOneBy(['code' => $this->getParentCode()]);
    $this->setParent($parent);
  }
  
  protected function getEntityManager()
  {
    return // ... return EntityManager
  }
}