public function getEmptyValueForLocalKey(string $relation)
    {
        if ($this->isPolymorphic($relation)) {
            $key = $this->localForeignKeys[$relation];

            return [
                $key['type'] => null,
                $key['id']   => null,
            ];
        }

        if ($this->isManyRelationship($relation)) {
            return [];
        }
    }