Rails:include不包含 [英] Rails :include doesn't include

问题描述

我的模特:

class Contact < ActiveRecord::Base
  has_many :addresses
  has_many :emails
  has_many :websites
  accepts_nested_attributes_for :addresses, :emails, :websites
  attr_accessible :prefix, :first_name, :middle_name, :last_name, :suffix,
                  :nickname, :organization, :job_title, :department, :birthday,
                  :emails_attributes
end

class Email < ActiveRecord::Base
  belongs_to :contact
  validates_presence_of :account
  validates_format_of :account, :with => /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\Z/i, :on => :create
  attr_accessible :contact_id, :account, :label
end

如果我运行以下查询，则会按预期返回emails:

c = Contact.find(3)
  Contact Load (3.2ms)  SELECT `contacts`.* FROM `contacts` LIMIT 1
=> #<Contact id: 3, prefix: nil, first_name: "Micah", middle_name: nil, last_name: "Alcorn", suffix: nil, nickname: nil, organization: nil, job_title: nil, department: nil, birthday: nil, created_at: "2011-07-04 23:50:04", updated_at: "2011-07-04 23:50:04">

c.emails
  Email Load (4.4ms)  SELECT `emails`.* FROM `emails` WHERE `emails`.`contact_id` = 3
=> [#<Email id: 3, contact_id: 3, account: "[e-mail]", label: "work", created_at: "2011-07-04 23:50:04", updated_at: "2011-07-04 23:50:04">]

但是，尝试建立:include关系不会:

c = Contact.find(3, :include => :emails)
  Contact Load (0.5ms)  SELECT `contacts`.* FROM `contacts` WHERE `contacts`.`id` = 3 LIMIT 1
  Email Load (0.8ms)  SELECT `emails`.* FROM `emails` WHERE `emails`.`contact_id` IN (3)
=> #<Contact id: 3, prefix: nil, first_name: "Micah", middle_name: nil, last_name: "Alcorn", suffix: nil, nickname: nil, organization: nil, job_title: nil, department: nil, birthday: nil, created_at: "2011-07-04 23:50:04", updated_at: "2011-07-04 23:50:04">

如您所见，正在执行SQL，但未返回电子邮件.我打算返回所有包含电子邮件的联系人，因此:joins不会有任何好处.我想念什么?

解决方案

电子邮件在那里.您尝试过c.emails吗?您会发现，电子邮件将在那里，而Rails不会执行其他数据库查询.

:include所做的事情被称为急切加载"，这基本上意味着Rails将尝试一种尽力而为的方法，即用对象的关系预填充对象，这样，当您实际请求该关系时，就不需要其他数据库查询了./p>

在此处查看"关联的快速加载"部分: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

您可能还需要查看以下RailsCast:
http://railscasts.com/episodes/181-include-vs-joins

My models:

class Contact < ActiveRecord::Base
  has_many :addresses
  has_many :emails
  has_many :websites
  accepts_nested_attributes_for :addresses, :emails, :websites
  attr_accessible :prefix, :first_name, :middle_name, :last_name, :suffix,
                  :nickname, :organization, :job_title, :department, :birthday,
                  :emails_attributes
end

class Email < ActiveRecord::Base
  belongs_to :contact
  validates_presence_of :account
  validates_format_of :account, :with => /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\Z/i, :on => :create
  attr_accessible :contact_id, :account, :label
end

If I run the following query, the emails are returned as expected:

c = Contact.find(3)
  Contact Load (3.2ms)  SELECT `contacts`.* FROM `contacts` LIMIT 1
=> #<Contact id: 3, prefix: nil, first_name: "Micah", middle_name: nil, last_name: "Alcorn", suffix: nil, nickname: nil, organization: nil, job_title: nil, department: nil, birthday: nil, created_at: "2011-07-04 23:50:04", updated_at: "2011-07-04 23:50:04">

c.emails
  Email Load (4.4ms)  SELECT `emails`.* FROM `emails` WHERE `emails`.`contact_id` = 3
=> [#<Email id: 3, contact_id: 3, account: "[e-mail]", label: "work", created_at: "2011-07-04 23:50:04", updated_at: "2011-07-04 23:50:04">]

However, attempting to :include the relationship does not:

c = Contact.find(3, :include => :emails)
  Contact Load (0.5ms)  SELECT `contacts`.* FROM `contacts` WHERE `contacts`.`id` = 3 LIMIT 1
  Email Load (0.8ms)  SELECT `emails`.* FROM `emails` WHERE `emails`.`contact_id` IN (3)
=> #<Contact id: 3, prefix: nil, first_name: "Micah", middle_name: nil, last_name: "Alcorn", suffix: nil, nickname: nil, organization: nil, job_title: nil, department: nil, birthday: nil, created_at: "2011-07-04 23:50:04", updated_at: "2011-07-04 23:50:04">

As you can see, the SQL is being executed, but the emails are not being returned. I intend to return all contacts with each containing email(s), so :joins won't do any good. What am I missing?

解决方案

The emails are there. Did you try c.emails? You will find that the emails will be there without Rails doing an additional DB query.

The thing that :include does is called eager loading, which basically means Rails will try a best effort method of prepopulating your objects with their relations, so that when you actually ask for the relation no additional DB queries are needed.

See the section "Eager loading of associations" here: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

You might also want to check out this RailsCast:
http://railscasts.com/episodes/181-include-vs-joins

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