Symfony2 - New entity was found through relationship - but it has been persisted

Question

I'm making some datafixtures for a Symfony project. I create entities in loops. The two entities are linked (SchoolClass is in School). But even if I persist School before linking SchoolClass to it, I still have this error:

[Doctrine\ORM\ORMInvalidArgumentException]
A new entity was found through the relationship 'Schoolit\SchoolBundle\SchoolClass#school' that was not configured to cascade persist operations for entity: 337 - Paul & Sons. To solve this issue: Either explicitly call EntityManager#persist() on this unknown entity or
configure cascade persist this association in the mapping for example @ManyToOne(..,cascade={"persist"}).

`

/**
 * {@inheritDoc}
 */
public function load(ObjectManager $manager)
{
    set_time_limit(10*60); // Setting max time to 10 minutes
    ini_set("memory_limit","1G"); // Setting memory limit to 1 Go

    if($this->environment == 'dev')
    {
        $faker = Factory::create('fr_FR');

        // -------------------------
        // Load developpement data
        // ------------------------

        // SCHOOLS
        // --------------------
        $schools = array();
        for($i=0; $i<30; $i++)
        {
            $entity = new School();
            $entity->setName($faker->company);
            $manager->persist($entity);
            $schools[] = $entity;
        }
        $manager->flush();

        // CLASSES
        // --------------------
        for($i=0; $i<100; $i++)
        {
            $entity = new SchoolClass();
            $entity->setSchool($schools[array_rand($schools)]);
            $manager->persist($entity);

            // To clear memory
            $manager->flush();
            $manager->clear();
        }

        $manager->flush();

    }
}

`

I'm watching the database, but even if there is no error, no row is inserted before the end of the full script. It is the normal behavior of $manager->flush() ? What can I do about it ?