private function createReference(RelationshipMetadata $relMeta, Model $model)
    {
        $reference = [];
        $identifier = $this->getIdentifierDbValue($model->getId());
        if (true === $relMeta->isPolymorphic()) {
            $reference[Persister::IDENTIFIER_KEY]  = $identifier;
            $reference[Persister::POLYMORPHIC_KEY] = $model->getType();
            return $reference;
        }
        return $identifier;
    }