Question

我在这里错过了什么吗？我阅读了关于加载加入的文档，并尝试了以下内容。您可以看到，当我使用＆＃34; include＆＃34;时，包含的连接不会输出所需的数据，但是当我在事后明确调用连接模型时，您可以看到存在所需的数据。以下是我在IRB中的模型和行为：

class Appearance < ActiveRecord::Base
  belongs_to :photo
  belongs_to :person
end


class Photo < ActiveRecord::Base
  belongs_to :event
  has_many :appearances, order: 'position'
  has_many :people, :through => :appearances
end


class PhotosController < ApplicationController
  def index
    @event            = Event.find(params[:event])
    @photos           = @event.photos.includes(:people, :appearances)
    # this does not show any appearance records, however this does
    # @photos           = @event.photos.first.people
  end
end

#IN IRB
 Photo.find(51941, :include => [:people, :appearances])
  Photo Load (0.4ms)  SELECT `photos`.* FROM `photos` WHERE `photos`.`id` = 51941 LIMIT 1
  Appearance Load (0.3ms)  SELECT `appearances`.* FROM `appearances` WHERE `appearances`.`photo_id` IN (51941) ORDER BY position
  Person Load (0.2ms)  SELECT `people`.* FROM `people` WHERE `people`.`id` IN (12103)

 => #<Photo id: 51941, time_created: "165732-0500", copyright_notice: "", country_primary_location_name: "", date_created: "20110119", created_at: "2011-01-20 07:33:24", updated_at: "2011-01-20 21:17:41", event_id: 602, position: 102, visible: nil, filename: "RAS_4824.JPG", folder: "", image: nil, image_file_name: nil, image_content_type: nil, image_file_size: nil, image_updated_at: nil> 

1.9.3p194 :036 > Photo.find(51941).appearances
  Photo Load (0.4ms)  SELECT `photos`.* FROM `photos` WHERE `photos`.`id` = 51941 LIMIT 1
  Appearance Load (0.4ms)  SELECT `appearances`.* FROM `appearances` WHERE `appearances`.`photo_id` = 51941 ORDER BY position
 => [#<Appearance id: 73654, photo_id: 51941, person_id: 12103, created_at: "2011-01-20 07:33:24", updated_at: "2011-01-20 07:33:24", position: 1>]

Answer 1

我认为你误解了includes的观点。使用includes（或include选项查找）告诉rails在加载父进程的同时从DB加载关联。这可以在您的IRB会话中进行，您可以从以下两行中看到：

Appearance Load (0.3ms)  SELECT `appearances`.* FROM `appearances` WHERE `appearances`.`photo_id` IN (51941) ORDER BY position
Person Load (0.2ms)  SELECT `people`.* FROM `people` WHERE `people`.`id` IN (12103)

当您将find的结果输出到控制台时，看关联记录的事实只是inspect方法的结果，不（默认情况下）显示所有关联的数据（通常会有大量数据）。

当您输入Photo.find(51941).appearances时，您正在检查的不是照片，而是检查外观关联（数组），这就是您可以在IRB控制台中查看结果的原因。

如果您想测试include是否有效，请先将照片分配给变量：

photo = Photo.find(51941, :include => [:people, :appearances])
Photo Load (0.4ms)  SELECT `photos`.* FROM `photos` WHERE `photos`.`id` = 51941 LIMIT 1
Appearance Load (0.3ms)  SELECT `appearances`.* FROM `appearances` WHERE `appearances`.`photo_id` IN (51941) ORDER BY position
Person Load (0.2ms)  SELECT `people`.* FROM `people` WHERE `people`.`id` IN (12103)

您不仅应该看到照片的SQL语句，还应该看到相关人员和外观的SQL语句。这表明rails已经急切地加载了这些关联。

如果您随后获得与photo.appearances的关联，并且您不会看到任何SQL查询，因为该关联已经从数据库加载：

1.9.3p194 :036 > photo.appearances
=> [#<Appearance id: 73654, photo_id: 51941, person_id: 12103, created_at: "2011-01-20 07:33:24", updated_at: "2011-01-20 07:33:24", position: 1>]

如果不使用include，您会在结果之前看到SQL查询：

1.9.3p194 :036 > photo.appearances
Appearance Load (0.3ms)  SELECT `appearances`.* FROM `appearances` WHERE `appearances`.`photo_id` IN (51941) ORDER BY position
=> [#<Appearance id: 73654, photo_id: 51941, person_id: 12103, created_at: "2011-01-20 07:33:24", updated_at: "2011-01-20 07:33:24", position: 1>]

急切的装载没有装入铁轨

1 个答案: