Eager Loading、Model 方法、fields_for、each 和 N+1答案

【问题标题】：Eager Loading, Model methods, fields_for, each and N+1Eager Loading、Model 方法、fields_for、each 和 N+1
【发布时间】：2015-07-21 06:24:24
【问题描述】：

我有一个相当复杂的应用程序（超过 30 个表），它有一些持续存在的 N+1 问题，我推测这是因为我没有做一些“The Rails Way™”的事情

我将举一个更复杂的例子。这个包括四个表：clins, positions_tasks, tasks 和 labor_hours。

positions_tasks 是clins, positions（本示例不需要）和tasks 以及has_many :labor_hours 之间的三向多对多连接。 labour_hours 表有一个整数字段，用于表示一年中的每个月和一些其他数据。方法total_hours 将所有月份汇总为一年的总小时数。在临床视图中，它显示临床信息和所有关联任务的表[以及其他相关数据]，并汇总每个任务的小时数，has_many :labor_hours, :through => :positions_tasks。我急切地加载了所有相关的表，包括labor_hours，除了labor_hours，所有的N+1个问题都消失了。

代码 sn-ps 跟随。

clins_controller 的急切加载：

@clin = Clin.includes(:proposal).includes(:positions_tasks).includes(:tasks).includes(:labor_hours).includes(:wbss).find(params[:id])`

在 clins/_form.html.erb 中显示表格行：

    <tbody>
        <% @clin.tasks.distinct.each do |t| %>
          <%= f.fields_for :task, t do |builder| %>
            <%= render "tasks/task_row", f: builder %>
          <% end %>
        <% end %>
    </tbody>

_task_row 部分：

<tr>
    <td><%= f.object.wbs_line_item.wbs.wbs_title %></td>
    <td><%= f.object.wbs_line_item.wbs_line_item %></td>
    <td><%= f.object.description %></td>
    <td><%= f.object.labor_hours.distinct.each.sum(&:total_hours) %>
    <td><div id="jump">
      <%= link_to "Edit", {:controller => :tasks, :action => :edit, :id => f.object.id } %>
    </div></td>
</tr>

临床模型：

class Clin < ActiveRecord::Base
  nilify_blanks

  belongs_to :proposal

  belongs_to :parent, :class_name => "Clin"
  has_many :children, :class_name => "Clin"

  has_many :positions_tasks
  has_many :labor_hours, :through => :positions_tasks
  has_many :tasks, :through => :positions_tasks
  has_many :wbs_line_items, :through => :tasks
  has_many :wbss, :through => :wbs_line_items
  has_many :pws_line_items, :through => :wbs_line_items
  has_many :pwss, :through => :wbss
end

工时模型：

class LaborHours < ActiveRecord::Base
  nilify_blanks

  belongs_to :positions_task
  belongs_to :year

  has_one :proposal, :through => :positions_task
  has_many :valid_years, :through => :proposal, :source => :years

  def total_hours
    m1 + m2 + m3 + m4 + m5 + m6 + m7 + m8 + m9 + m10 + m11 + m12
  end
end

positionsTask 模型：

class PositionsTask < ActiveRecord::Base
  nilify_blanks

  belongs_to :task
  belongs_to :position
  belongs_to :clin

  has_many :labor_hours

  has_one :company, :through => :position
  has_one :proposal, :through => :clin
  has_one :wbs_line_item, :through => :task
  delegate :wbs, :to => :wbs_line_item

  delegate :pws_line_items, :to => :wbs_line_item
  delegate :pwss, :to => :wbs_line_item

  validates_presence_of :task
  validates_presence_of :position
  validates_presence_of :clin

  accepts_nested_attributes_for :labor_hours, allow_destroy: true
end

任务模型：

class Task < ActiveRecord::Base
  nilify_blanks

  belongs_to :wbs_line_item
  belongs_to :task_category

  has_many :positions_tasks

  has_many :labor_hours, :through => :positions_tasks
  has_many :positions, :through => :positions_tasks
  has_many :clins, :through => :positions_tasks
  has_many :proposals, :through => :positions_tasks

  delegate :wbs, :to => :wbs_line_item
  delegate :pws_line_items, :to => :wbs_line_item
  delegate :pwss, :to => :wbs

  accepts_nested_attributes_for :positions_tasks, allow_destroy: true
  accepts_nested_attributes_for :labor_hours, allow_destroy: true

  validates_associated :positions_tasks

end

GET 和 SQL 加载：

Started GET "/clins/11/edit" for 127.0.0.1 at 2015-07-20 17:48:49 -0400
Processing by ClinsController#edit as HTML
  Parameters: {"id"=>"11"}
  Clin Load (0.2ms)  SELECT  "clins".* FROM "clins" WHERE "clins"."id" = $1 LIMIT 1  [["id", 11]]
  Proposal Load (0.2ms)  SELECT "proposals".* FROM "proposals" WHERE "proposals"."id" IN (1)
  PositionsTask Load (0.4ms)  SELECT "positions_tasks".* FROM "positions_tasks" WHERE "positions_tasks"."clin_id" IN (11)
  Task Load (0.6ms)  SELECT "tasks".* FROM "tasks" WHERE "tasks"."id" IN (1, 2, 3, 5, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 20, 23, 24)
  LaborHours Load (1.1ms)  SELECT "labor_hours".* FROM "labor_hours" WHERE "labor_hours"."positions_task_id" IN (1, 2, 3, 6, 7, 8, 9, 10, 12, 13, 14, 18, 19, 20, 21, 23, 24, 25, 26, 27, 30, 35, 36, 37)
  WbsLineItem Load (0.5ms)  SELECT "wbs_line_items".* FROM "wbs_line_items" WHERE "wbs_line_items"."id" IN (310, 312, 314, 316, 317, 318, 319, 413, 320, 321, 322, 324, 325, 326, 327, 328, 330, 333, 334)
  Wbs Load (0.4ms)  SELECT "wbss".* FROM "wbss" WHERE "wbss"."id" IN (1)
  Clin Load (0.2ms)  SELECT "clins".* FROM "clins"
  Rendered tasks/_task_header.html.erb (0.0ms)
  LaborHours Load (0.4ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 1]]
  Rendered tasks/_task_row.erb (2.6ms)
  LaborHours Load (0.3ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 2]]
  Rendered tasks/_task_row.erb (1.7ms)
  LaborHours Load (0.2ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 3]]
  Rendered tasks/_task_row.erb (1.4ms)
  LaborHours Load (0.2ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 5]]
  Rendered tasks/_task_row.erb (1.3ms)
  LaborHours Load (0.2ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 6]]
  Rendered tasks/_task_row.erb (1.4ms)
  LaborHours Load (0.2ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 7]]
  Rendered tasks/_task_row.erb (1.5ms)
  LaborHours Load (0.2ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 8]]
  Rendered tasks/_task_row.erb (1.3ms)
  LaborHours Load (0.2ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 9]]
  Rendered tasks/_task_row.erb (1.3ms)
  LaborHours Load (0.4ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 10]]
  Rendered tasks/_task_row.erb (1.9ms)
  LaborHours Load (0.2ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 11]]
  Rendered tasks/_task_row.erb (1.5ms)
  LaborHours Load (0.4ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 12]]
  Rendered tasks/_task_row.erb (2.2ms)
  LaborHours Load (0.5ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 14]]
  Rendered tasks/_task_row.erb (2.6ms)
  LaborHours Load (0.4ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 15]]
  Rendered tasks/_task_row.erb (2.2ms)
  LaborHours Load (0.2ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 16]]
  Rendered tasks/_task_row.erb (1.5ms)
  LaborHours Load (0.3ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 17]]
  Rendered tasks/_task_row.erb (1.9ms)
  LaborHours Load (0.3ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 18]]
  Rendered tasks/_task_row.erb (1.6ms)
  LaborHours Load (0.3ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 20]]
  Rendered tasks/_task_row.erb (1.9ms)
  LaborHours Load (0.2ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 23]]
  Rendered tasks/_task_row.erb (1.6ms)
  LaborHours Load (0.3ms)  SELECT "labor_hours".* FROM "labor_hours" INNER JOIN "positions_tasks" ON "labor_hours"."positions_task_id" = "positions_tasks"."id" WHERE "positions_tasks"."task_id" = $1  [["task_id", 24]]
  Rendered tasks/_task_row.erb (1.9ms)
  Rendered clins/_form.html.erb (47.6ms)
  Rendered clins/_errors.html.erb (0.0ms)
  Rendered clins/edit.html.erb within layouts/application (48.6ms)
  Rendered layouts/_header.html.erb (60.5ms)
  Rendered layouts/_sidenav.html.erb (0.4ms)
  Rendered layouts/_footer.html.erb (0.0ms)
Completed 200 OK in 140ms (Views: 106.4ms | ActiveRecord: 8.8ms)

我认为正在发生的事情是急切加载在 _form.html.erb 中的 distinct.each 和 fields_for 中丢失了，因为它传递的是任务对象而不是 clin 对象，和/或对 total_hours 的调用会导致每个被调用对象的负载，但我不确定如何确定它是什么，也不知道如何解决它们。

如何在没有 N+1 加载的 labour_hours 的情况下为表中的每个任务提供总和 task.labor_hours.total_hours？

【问题讨论】：

Protip（这根本无助于解决您的问题）：您可以将多个参数传递给includes：Clin.includes(:proposal, :positions_tasks, :tasks, :labor_hours, :wbss).find(params[:id])
从现在开始我将使用该约定。谢谢！

标签： ruby-on-rails ruby ruby-on-rails-4

【解决方案1】：

我不确定，但我有一个理论。你有这个（我已经删除了刚刚与我们无关的includes 调用：

@clin = Clin.includes(:tasks).includes(:labor_hours).find(params[:id])

您在这里所做的是预先加载与每个 Clin 关联的 Tasks 和与每个 Clin 关联的 LaborHours — 到目前为止，一切都很好，但在您看来，您正在这样做（或多或少）：

@clin.tasks.distinct.each do |task|
  # inside the partial...
  task.labor_hours...
end

在这里，您不是在访问与每个 Clin 关联的 LaborHours（这是您预先加载的内容），而是在访问与每个 Clin 关联的每个 Task 关联的 LaborHours。要访问与每个 Clin 关联的 LaborHours，您必须这样做：

@clin.labor_hours.each do |labor_hour|
  # ...
end

但由于您正在渲染任务（而不仅仅是 LaborHours），我认为这不是您想要的。相反，你需要告诉 Rails 你想要预先加载二阶关联——即与 Tasks 关联的 LaborHours，而不是与 Clins 关联的 LaborHours — 通过将哈希传递给 includes：

@clin = Clin.includes(:tasks => :labor_hours).find(params[:id])

附：您还可以进行一些额外的改进 — 例如，看起来您实际上并没有使用来自 LaborHours 的任何属性，您实际上只是在使用 total_hours 列的总和。但是当你可以让数据库来做时，用 Ruby 计算总和是一种浪费。但是，这超出了此答案的范围。

【讨论】：

你摇滚。我将控制器更新为：@clin = Clin.includes(:proposal, :positions_tasks, :wbss).includes(:tasks => :labor_hours).find(params[:id])，N+1 已解决。
太棒了！你也可以Clin.includes(:proposal, :positions_tasks, :wbss, :tasks => :labor_hours).find(params[:id])
我试过Clin.includes(:proposal, :positions_tasks, :tasks => :labor_hours, :wbss).find(params[:id]) 并且NetBeans 不喜欢它，所以我做了另一个，但它在最后没问题，因为你输入了它。在旁注中，我必须在模型/迁移中添加什么才能让数据库为我做添加？