Persisting other entities inside preUpdate of Doctrine Entity Listener

For clarity I continue here the discussion started here.

Inside a Doctrine Entity Listener, in the preUpdate method (where I have access to both the old and new value of any field of the entity) I'm trying to persist an entity unrelated to the focal one.

Basically I have entity A, and when I change a value in one of the fields I want to write, in the project_notification table, the fields oldValue, newValue plus others.

If I don't flush inside the preUpdate method, the new notification entity does not get stored in DB. If I flush it I enter into a infinite loop.

This is the preUpdate method:

public function preUpdate(ProjectTolerances $tolerances, PreUpdateEventArgs $event)
{
    if ($event->hasChangedField('riskToleranceFlag')) {
    $project = $tolerances->getProject();                
    $em = $event->getEntityManager();
    $notification = new ProjectNotification();
    $notification->setValueFrom($event->getOldValue('riskToleranceFlag'));
    $notification->setValueTo($event->getNewValue('riskToleranceFlag'));
    $notification->setEntity('Entity'); //TODO substitute with the real one
    $notification->setField('riskToleranceFlag');
    $notification->setProject($project);
    $em->persist($notification);


    // $em->flush(); // gives infinite loop
    }
}

Googling a bit I discovered that you cannot call the flush inside the listeners, and here it's suggested to store the stuff to be persisted in an array, to flush it later in the onFlush. Nonetheless it does not work (and probably it should not work, as the instance of the listener class gets destroyed after you call the preUpdate, so whatever you store in as protected attribute at the level of the class gets lost when you later call the onFlush, or am I missing something?).

Here is the updated version of the listener:

class ProjectTolerancesListener
{
    protected $toBePersisted = [];

    public function preUpdate(ProjectTolerances $tolerances, PreUpdateEventArgs $event)
    {
        $uow = $event->getEntityManager()->getUnitOfWork();
//        $hasChanged = false;

        if ($event->hasChangedField('riskToleranceFlag')) {
        $project = $tolerances->getProject();                
        $notification = new ProjectNotification();
        $notification->setValueFrom($event->getOldValue('riskToleranceFlag'));
        $notification->setValueTo($event->getNewValue('riskToleranceFlag'));
        $notification->setEntity('Entity'); //TODO substitute with the real one
        $notification->setField('riskToleranceFlag');
        $notification->setProject($project);

        if(!empty($this->toBePersisted))
            {
            array_push($toBePersisted, $notification);
            }
        else
            {
            $toBePersisted[0] = $notification;
            }
        }
    }

    public function postFlush(LifecycleEventArgs $event)
    {
        if(!empty($this->toBePersisted)) {

            $em = $event->getEntityManager();

            foreach ($this->toBePersisted as $element) {

                $em->persist($element);
            }

            $this->toBePersisted = [];
            $em->flush();
        }
    }
}

Maybe I can solve this by firing an event from inside the listener with all the needed info to perform my logging operations after the flush...but:

1) I don't know if I can do it

2) It seems a bit an overkill

Thank you!

Don't use preUpdate, use onFlush - this allows you to access the UnitOfWork API & you can then persist entities.

E.g. (this is how I do it in 2.3, might be changed in newer versions)

    $this->getEntityManager()->persist($entity);
    $metaData = $this->getEntityManager()->getClassMetadata($className);
    $this->getUnitOfWork()->computeChangeSet($metaData, $entity);