Issues with preload and through-through associations

[tagliala]

Preload is broken on Rails 7 with through-through associations

# frozen_string_literal: true

class Student < ApplicationRecord
  include ChronoModel::TimeMachine

  belongs_to :school
  has_one :city, through: :school
  has_one :country, through: :city

  validates :name, presence: true
end

Rails 6

[2] pry(main)> Student.as_of(Time.now).includes(:school, :city, :country).map(&:country).all?
  Student Load (1.7ms)  SELECT "students".* FROM (SELECT "history"."students".* FROM "history"."students" WHERE ( '2022-05-19 11:14:38.961991'::timestamp <@ history.students.validity )) "students"
  School Load (0.4ms)  SELECT "schools".* FROM (SELECT "history"."schools".* FROM "history"."schools" WHERE ( '2022-05-19 11:14:38.961991'::timestamp <@ history.schools.validity )) "schools" WHERE "schools"."id" IN ($1, $2)  [["id", 1], ["id", 2]]
  City Load (0.3ms)  SELECT "cities".* FROM (SELECT "history"."cities".* FROM "history"."cities" WHERE ( '2022-05-19 11:14:38.961991'::timestamp <@ history.cities.validity )) "cities" WHERE "cities"."id" IN ($1, $2)  [["id", 1], ["id", 2]]
  Country Load (0.2ms)  SELECT "countries".* FROM (SELECT "history"."countries".* FROM "history"."countries" WHERE ( '2022-05-19 11:14:38.961991'::timestamp <@ history.countries.validity )) "countries" WHERE "countries"."id" IN ($1, $2)  [["id", 1], ["id", 2]]
=> true

Rails 7

3.1.2 :004 > Student.as_of(Time.now).includes(:school, :city, :country).map(&:country).all?
  Student Load (1.3ms)  SELECT "students".* FROM (SELECT "history"."students".* FROM "history"."students" WHERE ( '2022-05-19 11:14:09.421689'::timestamp <@ history.students.validity )) "students"
  School Load (0.5ms)  SELECT "schools".* FROM (SELECT "history"."schools".* FROM "history"."schools" WHERE ( '2022-05-19 11:14:09.421689'::timestamp <@ history.schools.validity )) "schools" WHERE "schools"."id" IN ($1, $2)  [["id", 1], ["id", 2]]
  City Load (0.6ms)  SELECT "cities".* FROM (SELECT "history"."cities".* FROM "history"."cities" WHERE ( '2022-05-19 11:14:09.421689'::timestamp <@ history.cities.validity )) "cities" WHERE "cities"."id" IN ($1, $2)  [["id", 1], ["id", 2]]
  City Load (0.2ms)  SELECT "cities".* FROM "cities" WHERE "cities"."id" IN ($1, $2)  [["id", 1], ["id", 2]]
  Country Load (0.2ms)  SELECT "countries".* FROM (SELECT "history"."countries".* FROM "history"."countries" WHERE ( '2022-05-19 11:14:09.421689'::timestamp <@ history.countries.validity )) "countries" WHERE "countries"."id" IN ($1, $2)  [["id", 1], ["id", 2]]
 => false 

Rails 6: diowa/ruby3-rails6-bootstrap-heroku@d748a7e
Rails 7: diowa/ruby3-rails7-bootstrap-heroku@88842de

As we can see, Rails 7 performs an extra load outside of the history on the table in the middle

[tagliala]

Ref: rails/rails#43137 rails/rails@605acb9

[tagliala]

https://github.com/rails/rails/blob/3872bc0e54d32e8bf3a6299b0bfe173d94b072fc/activerecord/lib/active_record/associations/preloader/association.rb#L93

The problem may be here

@associate is false because preload_scope.empty_scope? returns false, since preload_scope includes the asoftime scope