Composite_primary_key issue: extra query when part of primary key not selected

extra_query_when_part_of_pk_not_selected.rb

gem "composite_primary_keys", "7.0.13"
gem 'activerecord', '4.1.9'
gem 'sqlite3'

require "composite_primary_keys"
require 'active_record'
require 'logger'

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

# Schema

ActiveRecord::Schema.define do

  create_table "org", id: false, force: true do |t|
    t.integer  "n"
    t.string   "or_customer"
    t.string   "or_rcc_code"
    t.integer  "or_acct_mgr", precision: 38, scale: 0
  end

  create_table "salesman_names", id: false, force: true do |t|
    t.string "sm_code"
  end
   
end

# Models

class Org < ActiveRecord::Base
  self.table_name = "org"
  self.primary_key = [:or_customer, :or_rcc_code]
  belongs_to :account_manager_1,
  'class_name': :SalesmanName,
  foreign_key: :or_acct_mgr,
  primary_key: :sm_code
end

class SalesmanName < ActiveRecord::Base
  self.table_name = "salesman_names"
  self.primary_key = :sm_code
end

# Data

Org.create!([
    {n: 17, or_customer: "MEG", or_rcc_code: "ABC", or_acct_mgr: 204},
    {n: 18, or_customer: "MEG", or_rcc_code: "DEF", or_acct_mgr: 204},
  ])
SalesmanName.create!([
    {sm_code: 105},
    {sm_code: 204},
  ])

# Tests

puts
puts "----- Runnning"
puts
orgs = Org.
  select(:or_customer).
  select(:or_acct_mgr).
  includes(:account_manager_1).
  order(:n)
orgs.each do |org|
  puts "Loading relation for #{org.attributes.inspect}:"
  org.account_manager_1
end

puts
puts "----- Control experiment"
puts
orgs = Org.
  select(:or_customer).
  select(:or_rcc_code).    # Select added
  select(:or_acct_mgr).
  includes(:account_manager_1).
  order(:n)
orgs.each do |org|
  puts "Loading relation for #{org.attributes.inspect}:"
  org.account_manager_1
end

stdout

-- create_table("org", {:id=>false, :force=>true})
D, [2015-02-08T09:04:15.323113 #20034] DEBUG -- :    (0.2ms)  CREATE TABLE "org" ("n" integer, "or_customer" varchar(255), "or_rcc_code" varchar(255), "or_acct_mgr" integer) 
   -> 0.0038s
-- create_table("salesman_names", {:id=>false, :force=>true})
D, [2015-02-08T09:04:15.323531 #20034] DEBUG -- :    (0.1ms)  CREATE TABLE "salesman_names" ("sm_code" varchar(255))
   -> 0.0003s
D, [2015-02-08T09:04:15.329218 #20034] DEBUG -- :    (0.0ms)  begin transaction
D, [2015-02-08T09:04:15.332193 #20034] DEBUG -- :   SQL (0.1ms)  INSERT INTO "org" ("n", "or_acct_mgr", "or_customer", "or_rcc_code") VALUES (?, ?, ?, ?)  [["n", 17], ["or_acct_mgr", 204], ["or_customer", "MEG"], ["or_rcc_code", "ABC"]]
D, [2015-02-08T09:04:15.332355 #20034] DEBUG -- :    (0.0ms)  commit transaction
D, [2015-02-08T09:04:15.332567 #20034] DEBUG -- :    (0.0ms)  begin transaction
D, [2015-02-08T09:04:15.333002 #20034] DEBUG -- :   SQL (0.0ms)  INSERT INTO "org" ("n", "or_acct_mgr", "or_customer", "or_rcc_code") VALUES (?, ?, ?, ?)  [["n", 18], ["or_acct_mgr", 204], ["or_customer", "MEG"], ["or_rcc_code", "DEF"]]
D, [2015-02-08T09:04:15.333131 #20034] DEBUG -- :    (0.0ms)  commit transaction
D, [2015-02-08T09:04:15.333976 #20034] DEBUG -- :    (0.0ms)  begin transaction
D, [2015-02-08T09:04:15.334424 #20034] DEBUG -- :   SQL (0.1ms)  INSERT INTO "salesman_names" ("sm_code") VALUES (?)  [["sm_code", 105]]
D, [2015-02-08T09:04:15.334537 #20034] DEBUG -- :    (0.0ms)  commit transaction
D, [2015-02-08T09:04:15.334686 #20034] DEBUG -- :    (0.0ms)  begin transaction
D, [2015-02-08T09:04:15.334948 #20034] DEBUG -- :   SQL (0.0ms)  INSERT INTO "salesman_names" ("sm_code") VALUES (?)  [["sm_code", 204]]
D, [2015-02-08T09:04:15.335072 #20034] DEBUG -- :    (0.0ms)  commit transaction

----- Runnning

D, [2015-02-08T09:04:15.335697 #20034] DEBUG -- :   Org Load (0.1ms)  SELECT "org"."or_customer", "org"."or_acct_mgr" FROM "org"   ORDER BY "org"."n" ASC
D, [2015-02-08T09:04:15.337532 #20034] DEBUG -- :   SalesmanName Load (0.1ms)  SELECT "salesman_names".* FROM "salesman_names"  WHERE "salesman_names"."sm_code" IN (204)
Loading relation for {"or_customer"=>"MEG", "or_acct_mgr"=>204}:
Loading relation for {"or_customer"=>"MEG", "or_acct_mgr"=>204}:
D, [2015-02-08T09:04:15.338680 #20034] DEBUG -- :   SalesmanName Load (0.1ms)  SELECT  "salesman_names".* FROM "salesman_names"  WHERE "salesman_names"."sm_code" = 204 LIMIT 1

----- Control experiment

D, [2015-02-08T09:04:15.339150 #20034] DEBUG -- :   Org Load (0.1ms)  SELECT "org"."or_customer", "org"."or_rcc_code", "org"."or_acct_mgr" FROM "org"   ORDER BY "org"."n" ASC
D, [2015-02-08T09:04:15.339724 #20034] DEBUG -- :   SalesmanName Load (0.1ms)  SELECT "salesman_names".* FROM "salesman_names"  WHERE "salesman_names"."sm_code" IN (204)
Loading relation for {"or_customer"=>"MEG", "or_rcc_code"=>"ABC", "or_acct_mgr"=>204}:
Loading relation for {"or_customer"=>"MEG", "or_rcc_code"=>"DEF", "or_acct_mgr"=>204}: