ActiveRecord的查询，包括协会数 [英] Activerecord query to include count of associations

问题描述

我有一个模型，用户，它有很多帖子。要查询前5个用户，我做的：

I have a model User and it has many posts. To query first 5 users, I do:

User.all :limit => 5

它返回一个用户，这是很好的，但越来越＃帖子每个用户的计数时，它再次查询数据库：

which returns a user which is good, but when getting the count of # of posts each user has, it queries the DB again:

a_user.posts.count

有没有一种方法可以让我避免这种情况的查询，并获得数与第一个查询？我是通过用户和IT组，每组一次查询用户的帖子的计数循环。

Is there a way I can avoid this query and get the count with the first query ? I am looping through users and it blocks each time it query for user's post's count.

推荐答案

的Rails有一个内置的方法，有宣称就像has_many关联时，必须设置为true，而且，你必须将列添加到数据库中。

Rails has a built in method, that has to be set to true when declaring associations like has_many, and also, you must add a column to the database.

设置 counter_cache：真正的在声明的has_many 协会

添加＃{table_name的} _count 到数据库中，它会增加或减少自动，这样你就可以直接在查询前五用户选择此列。

add #{table_name}_count to the database and it will be incremented and decremented automatically, so you can select this column directly when you query for the first five users.

这更多信息：

的http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

http://railscasts.com/episodes/23-counter-cache-column

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