sql - 当一个包罗万象的 SELECT 已经运行时，为什么要运行单个 SELECT 查询？ (Rails/ActiveRecord)

Question

我有以下代码(注意 includes 和 .each ):

subscribers = []
mailgroup.mailgroup_members.opted_to_receive_email.includes(:roster_contact, :roster_info).each { |m|
  subscribers << { :EmailAddress => m.roster_contact.member_email,
                   :Name => m.roster_contact.member_name,
                   :CustomFields => [ { :Key => 'gender', 
                                        :Value => m.roster_info.gender.present? ? m.roster_info.gender : 'X' 
                                    } ] 
                  } if m.roster_contact.member_email.present?
}
subscribers

相应地，我在日志中看到以下内容(即 select * from ROSTER_INFO ... IN (...) ):

SELECT `ROSTER_INFO`.* FROM `ROSTER_INFO` WHERE `ROSTER_INFO`.`ID` IN ('1450', '1000', '1111')

然而紧随其后的是select * from ROSTER_INFO对于已在 IN 中指定的每个 ID上一个查询的列表:

RosterInfo Load (84.8ms)  SELECT `ROSTER_INFO`.* FROM `ROSTER_INFO` WHERE `ROSTER_INFO`.`ID` = '1450' LIMIT 1
RosterInfo Load (59.2ms)  SELECT `ROSTER_INFO`.* FROM `ROSTER_INFO` WHERE `ROSTER_INFO`.`ID` = '1000' LIMIT 1
RosterInfo Load (56.8ms)  SELECT `ROSTER_INFO`.* FROM `ROSTER_INFO` WHERE `ROSTER_INFO`.`ID` = '1111' LIMIT 1

如 select *已经在 ROSTER_INFO 上完成了在所有感兴趣的 ID ( IN (...) ) 上，为什么另一个 select *为每个相同的 ID 再次完成？没有 ActiveRecord已经知道了ROSTER_INFO每个 ID 的列？

(同时，没有对 ROSTER_CONTACT 的单独查询，但是如果我从 :roster_contact 方法中删除 includes，那么 ROSTER_INFO 不会再次被查询，但 ROSTER_CONTACT 是。)

名册信息模型(节略)

class RosterInfo < ActiveRecord::Base
  self.primary_key = 'ID'
end

名册联系人模型(节略)

class RosterContact < ActiveRecord::Base
  self.primary_key = 'ID'

  has_many :mailgroup_members, foreign_key: 'rosterID'
  has_many :mailgroups, through: :mailgroup_members

  has_one :roster_info, foreign_key: 'ID'     # can use this line
  #belongs_to :roster_info, foreign_key: 'ID' # or this with no difference

  def member_name                             # I added this method to this
    roster_info.member_name                   # question only *after* having
  end                                         # figured out the problem.
end

RosterWeb 模型(节略)

class RosterWeb < ActiveRecord::Base
  self.primary_key = 'ID'
end

邮件组模型(节略)

class Mailgroup < ActiveRecord::Base
  self.primary_key = 'ID'

  has_many :mailgroup_members, foreign_key: 'mailCatID'

  has_one :mailing_list, foreign_key: :legacy_id
end

邮件组成员模型(节略)

class MailgroupMember < ActiveRecord::Base
  self.primary_key = 'ID'

  belongs_to :mailgroup, foreign_key: 'mailCatID'
  belongs_to :roster_contact, foreign_key: 'rosterID'
  belongs_to :roster_info, foreign_key: 'rosterID'
  belongs_to :roster_web, foreign_key: 'rosterID'

  scope :opted_to_receive_email, joins(:roster_web).where('ROSTER_WEB.receiveEmail=?', 1)
end

Answer 1

原来这个问题与m.roster_contact.member_name有关-- 不幸的是我做了 member_name roster_contact的方法本身(间接)查询 roster_info.member_name .我通过改变线路解决了这个问题

:Name => m.roster_contact.member_name,

直接查询roster_info如下

:Name => m.roster_info.member_name,

我很抱歉给您带来麻烦!