public function getTaxonomyKey()
    {
        $called_class_segments = explode('\\', get_called_class());
        $class_name = end($called_class_segments);
        return (is_null($this->taxonomy_key))
            ? Str::machine(Str::camelToHuman($class_name), Base::SEPARATOR)
            : $this->taxonomy_key;
    }