Why are individual SELECT queries running when an all-encompassing SELECT already ran? (Rails/ActiveRecord)

Question

I have the following code (note the includes and the .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

Correspondingly, I see the following in my logs (i.e. select * from ROSTER_INFO ... IN (...)):

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

Yet immediately after that there are select * from ROSTER_INFO for each ID already specified in the IN list of the previous query:

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

If select * had already been done on ROSTER_INFO on all IDs of interest (IN (...)), why is another select * being done again for each of the same IDs? Doesn't ActiveRecord already know all the ROSTER_INFO columns for each ID?

(Meanwhile, there are no individual queries for ROSTER_CONTACT, yet if I remove :roster_contact from the includes method, then ROSTER_INFO is not queried again, but ROSTER_CONTACT is.)

---

RosterInfo model (abridged)

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

RosterContact model (abridged)

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 model (abridged)

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

Mailgroup model (abridged)

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

  has_many :mailgroup_members, foreign_key: 'mailCatID'

  has_one :mailing_list, foreign_key: :legacy_id
end

MailgroupMember model (abridged)

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

The issue turned out to be related to m.roster_contact.member_name -- unfortunately I made member_name a method of roster_contact that itself (indirectly) queried roster_info.member_name. I resolved this by changing the line

:Name => m.roster_contact.member_name,

to directly query roster_info as follows

:Name => m.roster_info.member_name,

I am sorry for the trouble!