导轨中的多个表连接

Question

如何将下面的mysql查询写入rails activerecord

select
    A.*,
    B.* 
from
    raga_contest_applicants_songs AS A 
    join
        raga_contest_applicants AS B 
        ON B.contest_applicant_id = A.contest_applicant_id 
    join
        raga_contest_rounds AS C 
        ON C.contest_cat_id = B.contest_cat_id 
WHERE
    C.contest_cat_id = contest_cat_id 
GROUP BY
    C.contest_cat_id    

我知道如何在两个表上编写连接；但是，我对如何在 3 个表上使用 join 不是很有信心。

Answer 1

要重写您在问题中得到的 SQL 查询，我认为应该如下所示（尽管我很难完全可视化您的模型关系，所以这有点猜测）：

RagaContextApplicantsSong.
  joins(:raga_contest_applicants => [:raga_content_rounds], :contest_cat).
  group('raga_contest_rounds.contest_cat_id')

...这样joins 方法处理两个连接以及WHERE 子句，最后是group 调用。

更多供参考：

如果您将多个关联加入到同一模型you can simply list them：

Post.joins(:category, :comments)
Returns all posts that have a category and at least one comment

如果您要加入嵌套表，您可以将它们列在哈希表中：

Post.joins(:comments => :guest)
Returns all comments made by a guest

嵌套关联，多级：

Category.joins(:posts => [{:comments => :guest}, :tags])
Returns all posts with their comments where the post has at least one comment made by a guest

您还可以链接 ActiveRecord 查询接口调用，这样：

Post.joins(:category, :comments)
...produces the same SQL as...
Post.joins(:category).joins(:comments)

如果所有其他方法都失败了，您始终可以 pass a SQL fragment directly into the joins method 作为垫脚石，让您的工作查询更以 ARQI 为中心

   Client.joins('LEFT OUTER JOIN addresses ON addresses.client_id = clients.id')
=> SELECT clients.* FROM clients LEFT OUTER JOIN addresses ON addresses.client_id = clients.id