如何在Rails 3中加入表并计算记录?

问题描述

我有一个 Collection 类，里面有很多硬币.我正在尝试选择包含两个以上硬币的收藏.目前，我通过直接的 Ruby 做到这一点没有问题，但效率极低.

I have a Collection class which has many coins.I am trying to select collections which have more than two coins.Currently, I have no problem doing that through straight Ruby, but that's extremely inefficient.

我当前的代码:

collections = Collection.all.select { |c| c.coins.count > 2 }

我如何通过与 Arel 的 joins 调用来实现这一点?

How do I achieve that through a joins call with Arel?

谢谢！

推荐答案

回答我自己的问题:

Collection.joins(:coins).group("coins.collection_id").having("count(coins.id) > 2")

给 KJF 的帽子提示，他们询问了 这个类似的问题和krakover 回答它.

Hat tip to KJF who asked this similar question and to krakover for answering it.

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