Rails：包含多态关联

Question

我读到了这篇关于Using Polymorphism to Make a Better Activity Feed in Rails 的有趣文章。

我们最终会得到类似的东西

class Activity < ActiveRecord::Base
  belongs_to :subject, polymorphic: true
end

现在，如果其中两个主题是：

class Event < ActiveRecord::Base
  has_many :guests
  after_create :create_activities
  has_one :activity, as: :subject, dependent: :destroy 
end

class Image < ActiveRecord::Base
  has_many :tags
  after_create :create_activities
  has_one :activity, as: :subject, dependent: :destroy 
end

将 create_activities 定义为

def create_activities
  Activity.create(subject: self)
end

客人和标签定义为：

class Guest < ActiveRecord::Base
  belongs_to :event
end

class Tag < ActiveRecord::Base
  belongs_to :image
end

如果我们查询最近记录的 20 个活动，我们可以这样做：

Activity.order(created_at: :desc).limit(20)

我们有第一个可以解决的 N+1 查询问题：

Activity.includes(:subject).order(created_at: :desc).limit(20)

但是，当我们调用客人或标签时，我们又遇到了一个N​​+1查询问题。

为了能够使用分页，解决这个问题的正确方法是什么？

Answer 1

编辑 2：我现在使用的是 rails 4.2，并且急切加载多态性现在是一项功能 :)

编辑：这似乎在控制台中工作，但由于某种原因，我建议使用下面的部分仍然会生成带有子弹 gem 的 N+1 查询堆栈警告。我需要调查...

好的，我找到了解决方案（[编辑] 还是我？），但它假定您知道所有主题类型。

class Activity < ActiveRecord::Base
  belongs_to :subject, polymorphic: true

  belongs_to :event, -> { includes(:activities).where(activities: { subject_type: 'Event' }) }, foreign_key: :subject_id
  belongs_to :image, -> { includes(:activities).where(activities: { subject_type: 'Image' }) }, foreign_key: :subject_id
end

现在你可以做

Activity.includes(:part, event: :guests, image: :tags).order(created_at: :desc).limit(10)

但要使急切加载工作，您必须使用例如

activity.event.guests.first

而不是

activity.part.guests.first

所以你可能可以定义一个方法来代替主题

def eager_loaded_subject
  public_send(subject.class.to_s.underscore)
end

所以现在你可以查看

render partial: :subject, collection: activity

部分与

# _activity.html.erb
render :partial => 'activities/' + activity.subject_type.underscore, object: activity.eager_loaded_subject

还有两个（虚拟）部分

# _event.html.erb
<p><%= event.guests.map(&:name).join(', ') %></p>

# _image.html.erb
<p><%= image.tags.first.map(&:name).join(', ') %></p>

Answer 2

这有望在 rails 5.0 中得到修复。已经有一个问题和一个拉取请求。

https://github.com/rails/rails/pull/17479

https://github.com/rails/rails/issues/8005

我已经分叉了导轨并将补丁应用于 4.2-stable，它对我有用。尽管我不能保证定期与上游同步，但请随意使用我的 fork。

https://github.com/ttosch/rails/tree/4-2-stable

Answer 3

您可以使用ActiveRecord::Associations::Preloader 预加载分别链接到每个事件和图像对象的客人和标签，这些对象和图像对象作为主题与活动集合相关联。

class ActivitiesController < ApplicationController
  def index
    activities = current_user.activities.page(:page)

    @activities = Activities::PreloadForIndex.new(activities).run
  end
end

class Activities::PreloadForIndex
  def initialize(activities)
    @activities = activities
  end

  def run
    preload_for event(activities), subject: :guests
    preload_for image(activities), subject: :tags
    activities
  end

  private

  def preload_for(activities, associations)
    ActiveRecord::Associations::Preloader.new.preload(activities, associations)
  end

  def event(activities)
    activities.select &:event?
  end

  def image(activities)
    activities.select &:image?
  end
end

Answer 4

image_activities = Activity.where(:subject_type => 'Image').includes(:subject => :tags).order(created_at: :desc).limit(20)
event_activities = Activity.where(:subject_type => 'Event').includes(:subject => :guests).order(created_at: :desc).limit(20)
activities = (image_activities + event_activities).sort_by(&:created_at).reverse.first(20)

Answer 5

我建议将多态关联添加到您的 Event 和 Guest 模型中。 polymorphic doc

class Event < ActiveRecord::Base
  has_many :guests
  has_many :subjects
  after_create :create_activities
end

class Image < ActiveRecord::Base
  has_many :tags
  has_many :subjects
  after_create :create_activities
end

然后尝试做

Activity.includes(:subject => [:event, :guest]).order(created_at: :desc).limit(20)

Answer 6

这会生成有效的 SQL 查询，还是因为 events 不能与 tags 和 images 和 JOINed 和 guests 一起导致 events 失败？

class Activity < ActiveRecord::Base
  self.per_page = 10

  def self.feed
    includes(subject: [:guests, :tags]).order(created_at: :desc)
  end
end

# in the controller

Activity.feed.paginate(page: params[:page])

这将使用will_paginate。