public function addTerm($term_id) {
    $term = ($term_id instanceof Term) ? $term_id : Term::findOrFail($term_id);

    $term_relation = [
      'term_id' => $term->id,
      'vocabulary_id' => $term->vocabulary_id,
    ];

    return $this->related()->save(new TermRelation($term_relation));
  }