渴望加载关联计数与Arel（Rails 3） [英] Eager-loading association count with Arel (Rails 3)

问题描述

简单的任务：假设一篇文章有​​很多评论，则可以在长长的文章列表中显示每篇文章有多少评论。我正在尝试找出如何使用Arel预加载此数据。

Simple task: given that an article has many comments, be able to display in a long list of articles how many comments each article has. I'm trying to work out how to preload this data with Arel.

自述文件文件似乎在讨论这种情况，但它并未提供示例代码，也未提供方法在两个查询中而不是一个联合查询中执行，这会降低性能。

The "Complex Aggregations" section of the README file seems to discuss that type of situation, but it doesn't exactly offer sample code, nor does it offer a way to do it in two queries instead of one joined query, which is worse for performance.

给出以下内容：

class Article
  has_many :comments
end

class Comment
  belongs_to :article
end

如何为文章预载设置每个评论有多少？

How can I preload for an article set how many comments each has?

推荐答案

您可以使用SQL进行讨厌的操作，例如：

You can do something nasty using SQL like:

default_scope :select => 'articles.*, (select count(comments.id) from comments where comments.article_id = articles.id) as count_comments'

，然后您就可以访问Article.first.count_comments。

and then you would have access to Article.first.count_comments.

另一种（更精细的）方法是使用 counter_cache功能/ a from 属于关联。

Another (nicer) method to do it is to use the 'counter_cache' feature/option from belongs_to association.

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