// Inside of my Entity
 public function addEntertainmentFeature(\My\Entity\PropertyEntertainmentFeatures $entertainment){
         $registry = new \My\Entity\PropertyEntertainmentFeaturesRegistry();
         $registry->setProperty($this);
         $registry->setFeature($entertainment);
         $this->getEntertainment()->add($registry);
    }
// From my Controller
            // $checked is an array which contains the ID of all checkboxes in my form which are checked.
            foreach($checked as $key=>$value){
            if(!$entity->getPropertyEntertainmentFeatures()->contains( $this->em->getReference('\My\Entity\PropertyEntertainmentFeatures', $value) )){
                $entity->addEntertainmentFeature( $this->em->getReference('\My\Entity\PropertyEntertainmentFeatures', $value) );
                $this->em->persist($entity);
                $this->em->persist($entity->getEntertainment()->last());
            }
        }