列出重复值的数量 [英] List number of duplicate values

问题描述

我有以下Rails ActiveRecord关联，其中一课有很多书籍，而书籍属于一课。我想运行一个SQL命令，该命令将告诉我有多少课属于该课程。

I have the following a Rails ActiveRecord association where a lesson has_many books, and books belong_to a lesson. I want to run a SQL command that will tell me how many lessons have multiple books that belong to that lesson.

例如：第1课有第1本书和第2本书，我想看看发生了多少次。

Ex: Lesson 1 has books 1 and 2, and I want to see how many times that happens.

推荐答案

＃having 可让您执行＃group 过滤掉只有一本书的课程。

#having will let you do a #group that filters out Lessons that only have one Book.

Book.group(:lesson_id).having("count(lesson_id) > 1").count

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