Rails的关联HAS_ONE最新记录 [英] Rails Associations has_one Latest Record

问题描述

我有以下型号：

class Section < ActiveRecord::Base
  belongs_to :page
  has_many :revisions, :class_name => 'SectionRevision', :foreign_key => 'section_id'
  has_many :references

  has_many :revisions, :class_name => 'SectionRevision', 
                       :foreign_key => 'section_id'

  delegate :position, to: :current_revision

  def current_revision
    self.revisions.order('created_at DESC').first
  end
end

其中， current_revision 是最近创建的修改。是否有可能把 current_revision 成一个协会这样我就可以执行如查询Section.where（current_revision.parent_section_id ='1'）？或者我应该一个 current_revision 列添加的努力几乎或通过关联创建到我的数据库呢？

Where current_revision is the most recently created revision. Is it possible to turn current_revision into an association so I can perform query like Section.where("current_revision.parent_section_id = '1'")? Or should I add a current_revision column to my database instead of trying to create it virtually or through associations?

推荐答案

我明白你想获得其中每个部分的最后修订有部分的parent_section_id = 1;

I understand you want to get the sections where the last revision of each section has a parent_section_id = 1;

我也有类似的情况，首先，这是SQL（请想想的类别为您的部分，员额修订和USER_ID为parent_section_id -sorry，如果我不动了code你的需要，但我必须去）：

I have a similar situation, first, this is the SQL (please think the categories as sections for you, posts as revisions and user_id as parent_section_id -sorry if I don't move the code to your need but I have to go):

SELECT categories.*, MAX(posts.id) as M
FROM `categories` 
INNER JOIN `posts` 
ON `posts`.`category_id` = `categories`.`id` 
WHERE `posts`.`user_id` = 1
GROUP BY posts.user_id
having M = (select id from posts where category_id=categories.id order by id desc limit 1)

这是Rails中查询：

And this is the query in Rails:

Category.select("categories.*, MAX(posts.id) as M").joins(:posts).where(:posts => {:user_id => 1}).group("posts.user_id").having("M = (select id from posts where category_id=categories.id order by id desc limit 1)")

这个工作，这是丑陋的，我觉得最好的方法就是剪查询，但如果你有太多的部分，这将是一个问题，而循环槽他们。你也可以把这个查询转换为静态方法，而且，你的第一个想法，有你的部分表内的REVISION_ID将有助于优化查询，但会下降正常化（有时是必要的），你将不得不当一个新的修订该节创建更新此字段（因此，如果你要挣很多版本中修改一个庞大的数据库，也许这将是一个糟糕的主意，如果你有一个缓慢的服务器...）

This works, it is ugly, I think the best way is to "cut" the query, but if you have too many sections that would be a problem while looping trough them; you can also place this query into a static method, and also, your first idea, have a revision_id inside of your sections table will help to optimize the query, but will drop normalization (sometimes it is needed), and you will have to be updating this field when a new revision is created for that section (so if you are going to be making a lot of revisions in a huge database it maybe would be a bad idea if you have a slow server...)

更新 我回来嘿嘿，我做了一些测试，并检查了这一点：

UPDATE I'm back hehe, I was making some tests, and check this out:

def last_revision
    revisions.last
end

def self.last_sections_for(parent_section_id)
  ids = Section.includes(:revisions).collect{ |c| c.last_revision.id rescue nil }.delete_if {|x| x == nil}

  Section.select("sections.*, MAX(revisions.id) as M")
         .joins(:revisions)
         .where(:revisions => {:parent_section_id => parent_section_id})
         .group("revisions.parent_section_id")
         .having("M IN (?)", ids)
end

我做了这个查询，并与我的表（希望我命名好参数，可以是从以前一样Rails的查询，但我改变了其优化的查询）工作;注意组;在包括使得它在大型数据集最优的，而抱歉，我无法找到一个方法，使与HAS_ONE的关系，但我会用这个去的，也重新考虑你提到在一开始的领域。

I made this query and worked with my tables (hope I named well the params, it is the same Rails query from before but I change the query in the having for optimization); watch out the group; the includes makes it optimal in large datasets, and sorry I couldn't find a way to make a relation with has_one, but I would go with this, but also reconsider the field that you mention at the beginning.

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