ActiveRecordで多階層カテゴリ という投稿をしたところ、 @jnchito さんに Ancestry なるgemを教えていただきました。 stefankroes/ancestry Organise ActiveRecord model into a tree structure 準備 $ rails g model category name:string $ rake db:migrate mysql> desc categories; +------------+--------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +------------+--------------+------+-----+---------+---
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