symfony2 doctrine2 batch processing error with entity

i am trying to insert a lot of entities to DB. When i try to insert to example 5 entities with this code it works without problem... but when i implemented the batch method there it causes this error:

(code)

        if (!empty($invite_this_peopleArray)) {
            $batchSize = 20;
            $i = 0;
            foreach ($explodeInviteArray as $explodingUser) {
                ++$i;

                $notifiedUser=$userRepo->find($explodingUser);

                $notify=new Notify();
                $notify->setNotifyUser($user);
                $notify->setUser($notifiedUser);
                $notify->setStatus($lastStatus);
                $notify->setTyp('invite');
                $notify->setViewed(false);
                $notify->setAdInfo($name);

                $em->persist($notify);

                if (($i % $batchSize) === 0) {
                    $em->flush();
                    $em->clear(); // Detaches all objects from Doctrine!
                }            
            }
        }

Error:

A new entity was found through the relationship 'TB\NotifyBundle\Entity\Notify#notifyUser' that was not configured to cascade persist operations for entity: (nick of notify user user($user->getUsername())). 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"}).

What is the problem please?

BTW: when i modified the code so i clear only $notify and $notifiedUser like:

                if (($i % $batchSize) === 0) {
                    $em->flush();
                    $em->clear($notify); // Detaches all objects from Doctrine!
                    $em->clear($notifiedUser); // Detaches all objects from Doctrine!
                }   

The error is gone but i try to insert 4000 rows and i am getting insted this error:

Fatal error: Maximum execution time of 30 seconds exceeded in vendor/doctrine/orm/lib/Doctrine/ORM/UnitOfWork.php on line 514

When i try insert 1000 rows.

Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 32 bytes)

(so there is somewhere memory leak? )