private function handleInto(
        ModelId $into,
        RelationshipManager $relationships,
        ModelCollector $collector,
        ModelInterface $model
    ) {
        // If we have a null, it means insert into the tree root.
        if (0 === $into->getId() || null === $parent = $collector->getModel($into)) {
            $relationships->setRoot($model);
            return;
        }

        $relationships->setParent($model, $parent);
    }