ActiveRecord - 查询多态关联 [英] ActiveRecord - querying polymorphic associations

问题描述

我正在使用多态关联来跟踪我项目中的评论.所有非常简单的东西.

I am using polymorphic associations to track Comments in my project. All very straight forward stuff.

我遇到的问题是基于多态关联进行查询，并从 Comment 模型返回到它的所有者.

The problem I have is in querying based on the polymorphic association and joining from the Comment model back to it's owner.

所以……

我有一个评论模型

class Comment < ActiveRecord::Base
  belongs_to :commentable, :polymorphic => true
end

还有一个论坛主题模式:

And a ForumTopics mode:

class ForumTopic < ActiveRecord::Base
  has_many :comments, :as => :commentable
end

我还有其他几个可评论"的模型，它们现在并不重要.所有这些都有效.

I have several other "commentable" models that aren't important right now. All of this works.

我想要做的是找到属于具有指定条件的 ForumTopic 的所有评论(在本例中，'featured' == true).

What I am trying to do is find all of the Comments that belong to a ForumTopic with a specified condition (in this case, 'featured' == true).

当我尝试使用查找器加入模型时:

When I try and use a finder to join the models:

@comments = Comment.find(:all 
            :joins => :commentable
            :conditions => ["forum_topics.featured = ? ", true] 
            )

我收到以下错误:

不能急切地加载多态关联:commentable

使用 AR包含语法":

Using the AR "include syntax":

@comments = Comment.find(:all 
            :include => :forum_topics
            :conditions => ["forum_topics.featured = ? ", true] 
            )

返回:

未找到名为forum_topics"的关联；也许你拼错了?

如果我尝试使用表名而不是关联名(字符串而不是符号)加入:

If I try and join with a table name instead of the association name (string instead of symbol):

@comments = Comment.find(:all,
            :joins => "forum_topics",
            :conditions => ["forum_topics.featured = ? ", true] 
            )

我明白了:

Mysql::Error: Unknown table 'comments': SELECT comments. FROM comments forum_topics WHERE (forum_topics.featured = 1 )*

Mysql::Error: Unknown table 'comments': SELECT comments. FROM comments forum_topics WHERE (forum_topics.featured = 1 )*

(您可以在此处看到底层查询的语法完全关闭，并且完全没有连接).

(You can see here that the syntax of the underlying query is totally off and the join is missing altogether).

不确定我正在做的事情是否可行，还有其他方法可以实现所需的结果，但看起来应该是可行的.

Not sure if what I am doing is even possible, and there are other ways to achieve the required result but it seems like it should be doable.

有什么想法吗?我有什么遗漏吗?

Any ideas? Anything I am missing?

推荐答案

啊！

我想我发现了问题.

通过以下方式加入时:

@comments = Comment.find(:all,
        :joins => "forum_topics",
        :conditions => ["forum_topics.featured = ? ", true] 
        )

你需要整个加入！

:joins => "INNER JOIN forum_topics ON forum_topics.id = comments.commentable_id",

看到令人敬畏的:http://guides.rubyonrails.org/active_record_querying.html#joining-tables

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