如何在Rails中进行排序？ [英] How to order by in Rails?

问题描述

我正在研究一个小型的博客引擎。

有以下表格：Blog和Message。

博客有一个外键：last_message_id，所以我通过调用blog.last_message来访问博客中的最​​后一条消息。

我有下面的代码来使它工作：

  class Blog< ActiveRecord :: Base 
 belongs_to：last_message，：class_name => Message
 end 
  

我需要通过最后一条消息来订购博客。但是当我打电话给

$ $ p $ code> blogs.order（last_message.created_at DESC）

不起作用。我得到以下错误：

pre $ PGError：错误：缺少表last_message的FROM子句条目
ORDER BY last_messa ...


如何让它工作？













$ b $ p $ blogs.joins（：last_message）.order（messages.created_at DESC）。


解决方案

我认为你的模型是错误的。请参阅rails自动将2个属性添加到模型： created_at 和 update_at 。所以有一个像你描述的关系redondant。对我来说，它应该是这样的：

 ＃model / blog.rb 
 class Blog< ActiveRecord :: Base 
 has_many：messages 
 end 
 
＃model / message.rb 
 class Message< ActiveRecord :: Base 
 belongs_to：blog 
 end 
  

然后，博客订购的最后一条消息，你可以这样做：

  Blog.joins（：messages）.order（messages.created_at_desc ）
  

正如您可能已经注意到的那样，会为您的博客模型提供两倍的条目。如果这不是问题，请继续。如果是这样，你有两个选择：做一个每个并测试你是否已经看到博客 - 如果没有，你显示它。或者，你可以写你自己的SQL。

I'm working on a small blog engine.

There are the following tables: Blog and Message.

Blog has a foreign key: last_message_id, so I access the last message in the blog by calling blog.last_message

I have the following code to make it work:

class Blog < ActiveRecord::Base  
  belongs_to :last_message, :class_name => "Message"
end

I need to order the blogs by the last messages. But when I call

blogs.order("last_message.created_at DESC")

It doesn't work. I get the following error:

PGError: ERROR:  missing FROM-clause entry for table "last_message"
ORDER BY  last_messa...

How can I make it work?

UPDATE

Here's the solution:

blogs.joins(:last_message).order("messages.created_at DESC").

解决方案

I think your model is wrong. See rails automaticly add 2 attributes to a model : created_at and update_at. So having a relationship like you describe is redondant. To me, it should look like this :

#model/blog.rb
class Blog < ActiveRecord::Base
  has_many :messages
end

#model/message.rb
class Message < ActiveRecord::Base
  belongs_to :blog
end

Then, to get the blogs ordered by the last message, you could do this :

Blog.joins(:messages).order("messages.created_at_desc")

That as you may have noticed will give you double entries for your blog model. If that is not a problem, go ahead. If it is, you have two options : doing a each and test if you already saw the blog - if not, you display it. Or, you could write your own sql.

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