What is that for?

FavoriteRepository.php

<?php

use Doctrine\ORM\EntityRepository;

class FooRepository extends EntityRepository
{
    public function save(Foo $foo)
    {
        $em = $this->getEntityManager();
        $uof = $em->getUnitOfWork();
        
       if ($uof->isEntityScheduled($foo)) {
            throw new \RuntimeException(sprintf(
                'Entity %s is stored in database -> id = %d',
                Foo::class,
                $foo->getId()
            ));
        }

        $em->persist($foo);
        $em->flush();        
    }
}