Act as Taggable ってあるのねん。勉強勉強。

「This means that you must have a table solely for holding tag names. Usually, this table is named ‘tags’ having at least 2 columns: primary key (usually an autoincrement integer called ‘id’ - the AR standard for PKs) and a ‘name’ columns, usually a varchar. You must also have a defined ActiveRecord model class that relates to this table, by default called ‘Tag’. 」