これは超便利！指定のカラムをキーとしたHashを簡単に作る方法 - Qiita

user = User.all { [name: 'a', category: 1], [name: 'b', category: 2], [name: 'c', category: 1] } user.group_by(&:category) { 1 : [{name: 'a'}, {name: 'c'}], 2 : [{name: 'b'}] }