Rails :include 不包括

Question

我的模型：

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，emails 会按预期返回：

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: "not@real.address", label: "work", created_at: "2011-07-04 23:50:04", updated_at: "2011-07-04 23:50:04">]

但是，尝试:include 的关系不会：

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，但没有返回电子邮件。我打算返回所有包含每封电子邮件的联系人，所以:joins 不会有任何好处。我错过了什么？

Answer 1

电子邮件在那里。你试过c.emails吗？你会发现没有 Rails 做额外的数据库查询，邮件就在那里。

:include 所做的事情被称为预加载，这基本上意味着 Rails 将尽最大努力使用它们的关系预填充您的对象，因此当您实际请求关系时不需要额外的数据库查询。

请参阅此处的“渴望加载关联”部分： http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

您可能还想查看此 RailsCast：
http://railscasts.com/episodes/181-include-vs-joins