如何使用的has_many assocations零号的所有记录找到？ [英] How to find all records with zero number of has_many assocations?

问题描述

让我们假设你有问题模型如下的has_many 的关联：（例如从的这个插件）

Let's say you have Question model with following has_many association:(example taken from this plugin)

has_many :comment_threads, 
         :class_name => "Comment", 
         :as         => :commentable, 
         :dependent  => :destroy

我将如何定义一个范围或一个类的方法返回一个没有相关评论吗？

How would I define a scope or a class method that returns questions that has no associated comments?

基本上我想要 Question.unanswered 返回所有问题的零意见。

Basically I want Question.unanswered to return all questions with zero comments.

推荐答案

我觉得有办法 counter_cache 是更好和更快，但您可以创建你想要这样的范围（您可能需要一些调整，如果我猜的表或列名称错误）：

I think approach with counter_cache is nicer and faster, but you can create the scope you want like that (you might need some adjustments if I guessed tables or columns names wrong):

scope :unanswered, 
  joins('LEFT OUTER JOIN comments ON comments.commentable_id = questions.id').
  where('comments.id IS NULL')

使用 LEFT OUTER JOIN 生成连接表，其中一个未加注释的问题，意见表中的所有列设置为 NULL 。这些正是我们需要的，所以我们用它们进行过滤的行其中，。

Using LEFT OUTER JOIN generates joined table where for an uncommented question all columns of comments table are set to NULL. These are exactly the rows we need so we filter them using where.

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