public function getNonExistingRelated(array $relationships): array
    {
        $nonExisting = [];

        foreach ($relationships as $relation) {
            if ($this->hasAttribute($relation) && array_key_exists($relation, $this->relationships)) {
                $nonExisting = array_merge($nonExisting, $this->getNonExistingFromRelation($relation));
            }
        }

        return $nonExisting;
    }