public function validateIdentifier($id)
    {
        $printed = print_r($id, true);
        if (count($this->keys) > 1 && !is_array($id)) {
            throw new \RuntimeException("Entity $this->name has a composed key, finding by id requires an array, given: $printed");
        }
        if (count($this->keys) !== count($id)) {
            $keys = print_r($this->keys, true);
            throw new \RuntimeException("Entity $this->name requires the following keys: $keys, given: $printed");
        }
    }