急切地加载主动记录关联时避免双重查询？

Question

为了进行单个DB查询，我渴望加载帖子及其翻译数据（使用Rails 6和Mobility（*）），但它产生了2个SQL查询：

# app/models/post.rb
class Post < ApplicationRecord
  extend Mobility
  translates :title, :description, backend: :table
end

# app/controllers/posts_controller.rb
class PostsController < ApplicationRecord
  def index
    @posts = Post.eager_load(:translations).limit(3)
  end
end

<%# app/views/posts/index.html.erb %>
<% @posts.each do |post| %>
  <h1><%= post.title %></h1>
  <div><%= post.description %></div>
<% end %>

恢复：

首先，选择所有帖子ID
• 然后返回所有具有这些ID的帖子的属性

•  SELECT DISTINCT "posts"."id" FROM "posts"
      LEFT OUTER JOIN "post_translations" 
      ON "post_translations"."post_id" = "posts"."id" LIMIT $1  [["LIMIT", 3]]
  
    SELECT "posts"."id" AS t0_r0, "posts"."created_at" AS t0_r1, "posts"."updated_at" AS t0_r2, "post_translations"."id" AS t1_r0, "post_translations"."locale" AS t1_r1, "post_translations"."created_at" AS t1_r2, "post_translations"."updated_at" AS t1_r3, "post_translations"."title" AS t1_r4, "post_translations"."description" AS t1_r5, "post_translations"."post_id" AS t1_r6 FROM "posts" 
      LEFT OUTER JOIN "post_translations" 
      ON "post_translations"."post_id" = "posts"."id" 
      WHERE "posts"."id" IN ($1, $2, $3)  [["id", "00060a7d-b846-5fc5-a372-1fc3462c695c"], ["id", "008db504-6fb4-5e90-bdca-4293ebe6d920"], ["id", "034944c1-4067-5ae5-89aa-4777ef14d66b"]]

如何避免使用ID内存的Double SQL语句？

（*）笔记 Mobility文档

具有产生单个SQL语句的示例，但是正如Chris Salzberg所指出的那样，在此示例中根本不使用其查询API，因此不应是罪魁祸首。试图证明该问题可能与移动性无关，而是主动记录本身，以下是一个与移动性相同的代码，这显示了相同的双重问问题（NB：这只是出于演示目的，就像我一样想要继续使用移动性）：

---

class Post < ApplicationRecord
  has_many :translations, ->{ where(locale: I18n.locale) }

  %i(title description).each do |attr|
    define_method(attr) do
      translations.first.send(attr)
    end
  end

  class Translation < ApplicationRecord; end
end

<%# app/views/posts/index.html.erb %>
<% Post.eager_load(:translations).limit(3).each do |post| %>
  <h1><%= post.title %></h1>
  <div><%= post.description %></div>
<% end %>

如果您试图从收藏中获取一些非常具体的属性，那么CollectionProxy将为您提供单个查询。如果没有提供任何属性（列），则在执行

OUTER JOIN

查询之前进行不同的查询。

，我还没有阅读整个实施以确认其背后的推理。

但让我向你展示一件事。

2.5.1 :037 > Post.eager_load(:translations).each do |post|
2.5.1 :038 >     puts post.title
2.5.1 :039?>   end
  
  SQL (0.5ms)  SELECT "posts"."id" AS t0_r0, "posts"."title" AS t0_r1, "posts"."created_at" AS t0_r2, "posts"."updated_at" AS t0_r3, "post_translations"."id" AS t1_r0, "post_translations"."title" AS t1_r1, "post_translations"."content" AS t1_r2, "post_translations"."locale" AS t1_r3, "post_translations"."post_id" AS t1_r4, "post_translations"."created_at" AS t1_r5, "post_translations"."updated_at" AS t1_r6 FROM "posts" LEFT OUTER JOIN "post_translations" ON "post_translations"."post_id" = "posts"."id"

title 1
title 2
title 3
title 4

Answer 1

在上述情况下，您可以看到eager_load可以执行您的期望。我认为，在懒惰评估时，您认为它不提出所需属性的类似情况，除了查询

distinct

希望这有帮助。如果有任何评论，请发布，以便我可以澄清一下。 ：）

如果您以某种方式迫使Rails在单个查询中运行它，那么您将不喜欢后果。

由于

OUTER JOIN

2.5.1 :040 > Post.eager_load(:translations)
  SQL (0.3ms)  SELECT  DISTINCT "posts"."id" FROM "posts" LEFT OUTER JOIN "post_translations" ON "post_translations"."post_id" = "posts"."id" LIMIT ?  [["LIMIT", 11]]
  SQL (0.5ms)  SELECT "posts"."id" AS t0_r0, "posts"."title" AS t0_r1, "posts"."created_at" AS t0_r2, "posts"."updated_at" AS t0_r3, "post_translations"."id" AS t1_r0, "post_translations"."title" AS t1_r1, "post_translations"."content" AS t1_r2, "post_translations"."locale" AS t1_r3, "post_translations"."post_id" AS t1_r4, "post_translations"."created_at" AS t1_r5, "post_translations"."updated_at" AS t1_r6 FROM "posts" LEFT OUTER JOIN "post_translations" ON "post_translations"."post_id" = "posts"."id" WHERE "posts"."id" IN (?, ?, ?, ?)  [["id", 1], ["id", 2], ["id", 3], ["id", 4]]
 => #<ActiveRecord::Relation [#<Post id: 1, title: nil, created_at: "2021-07-08 12:42:13", updated_at: "2021-07-09 15:32:48">, #<Post id: 2, title: nil, created_at: "2021-07-09 15:33:50", updated_at: "2021-07-09 15:33:50">, #<Post id: 3, title: nil, created_at: "2021-07-09 15:33:55", updated_at: "2021-07-09 15:33:55">, #<Post id: 4, title: nil, created_at: "2021-07-09 15:33:57", updated_at: "2021-07-09 15:33:57">]>

，铁轨设法避免了

DISTINCT "posts"."id"

。拥有最后一个条件将迫使数据库完全比较记录，而不仅仅是主要键！

我100％确定：DISTINCT "posts"."*"

在引擎盖下，除非您跳过相应的索引，否则此请求恰好是索引。

Answer 2

distinct.id：

固定。*：