ActiveRecordで多階層カテゴリ - Qiitamysql> select * from categories; +----+---------+--------------------+---------------------+---------------------+ | id | name | parent_category_id | created_at | updated_at | +----+---------+--------------------+---------------------+---------------------+ | 1 | metal | NULL | 2014-12-27 09:38:14 | 2014-12-27 09:38:14 | | 2 | melodic | 1 | 2014-12-27 09:38:14 | 2014-12-27 09:38:14 | | 3 | black |
