h365info https://blog.hatena.ne.jp/h365info/ h365info's blog https://h365info.hatenablog.com/ h365 アダルト Rails Rails4 で group by して count を取りたい時にこんなふうに書けます。 本に対してレビューが書かれていて、本ごとのレビュー件数を取りたい場合は Review.group(:book_id).order('count_all desc').count こんなふうに書くと (0.3ms) SELECT COUNT(*) AS count_all, book_id AS book_id FROM `reviews` GROUP BY book_id ORDER BY count_all desc => {588=>5, 589=>2, 590=>1, 591=>0, 592=>4… 190 <iframe src="https://hatenablog-parts.com/embed?url=https%3A%2F%2Fh365info.hatenablog.com%2Fentry%2F2014%2F02%2F05%2F174950" title="Rails で group by して count を取りたい時のイディオム - h365info&#39;s blog" class="embed-card embed-blogcard" scrolling="no" frameborder="0" style="display: block; width: 100%; height: 190px; max-width: 500px; margin: 10px 0px;"></iframe> Hatena Blog https://hatena.blog 2014-02-05 17:49:50 rich https://h365info.hatenablog.com/entry/2014/02/05/174950 1.0 100%