public function getCollectionPersister() : CollectionPersister
    {
        if (! isset($this->collectionPersister)) {
            $pb                        = $this->getPersistenceBuilder();
            $this->collectionPersister = new Persisters\CollectionPersister($this->dm, $pb, $this);
        }
        return $this->collectionPersister;
    }