自动选择 ActiveRecord 中连接表的所有列的方法？

发布于 2024-11-10 02:26:31 字数 399 浏览 0 评论 0原文

想知道升级到 Arel 后，ActiveRecord 是否支持自动方式从连接表中选择列，而无需在 select 子句中显式列出它们。

鉴于我将 users 表加入到 posts，希望能够以更简洁的方式执行类似的操作：

  scope :select_user,
    select("posts.*, " + User.columns.map{|c| 
      "users.#{c.name} user_#{c.name}"}.join(','))

因此，我想自动选择字段来自用户，这样我就可以说

Post.joins(:user).select_user.first.user_name

原文

Wondering with the upgrade to Arel, if ActiveRecord yet supports an automated way to select in the columns from joined tables, without having to explicitly list them in a select clause.

Given I'm joining the users table to posts, Would like to be able to do something like this in a more succinct manner:

  scope :select_user,
    select("posts.*, " + User.columns.map{|c| 
      "users.#{c.name} user_#{c.name}"}.join(','))

So, I want to automatically select the fields from user so that I get say

Post.joins(:user).select_user.first.user_name

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表情可笑 2024-11-17 02:26:31

我认为如果您想在查询完成后访问关联的属性，那么您真正想要的是使用 includes 。因此，相同的查询将是

Post.includes(:user).first.user.name

这将导致两个查询，一个 select * from users 和一个 select * from posts where posts.user_id in (id1, id2, id3...)< /code> （假设post own_to user）在特定情况下可能会影响性能，但从语法的角度来看可能是值得的。在某些情况下，它甚至可能更快Rails :include 与 :joins。

如果您向查询添加限制运算符，Arel 还会智能地使用 join，例如 Post.includes(:user).where('users.name = "Bo Bob"') 将使用 join 执行一个查询，并且还使您能够在生成的 Post 对象上使用关联。

I think what you actually want here is to use includes if you want to access the association's attributes after the query is complete. So the same query would be

Post.includes(:user).first.user.name

This will result in two queries, one select * from users and one select * from posts where posts.user_id in (id1, id2, id3...) (assuming post belongs_to user) which may be a performance hit in a particular situation, but may be worth it from a syntax standpoint. In some cases it may even be faster Rails :include vs. :joins.

Arel will also intelligently use a join if you add a restriction operator to the query e.g. Post.includes(:user).where('users.name = "Bo Bob"') will do one query with a join and also enable you to use the associations on the resulting Post object.

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日暮斜阳 2024-11-17 02:26:31

如果我正确理解你的问题，你可以使用 'joins':

class Post < ActiveRecord::Base
  belongs_to :user

  scope :select_user, joins(:user).where('users.name = "Mary"')
  ...
end

或参数：

class Post < ActiveRecord::Base
  belongs_to :user

  scope :select_user, lambda {|user_name|
    joins(:user).where('users.name = ?', user_name)
  }
  ...
end

If I understand your question correctly, you can do it with 'joins':

class Post < ActiveRecord::Base
  belongs_to :user

  scope :select_user, joins(:user).where('users.name = "Mary"')
  ...
end

or with a param:

class Post < ActiveRecord::Base
  belongs_to :user

  scope :select_user, lambda {|user_name|
    joins(:user).where('users.name = ?', user_name)
  }
  ...
end

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