Autosave happens every time on has_one :through

[Kukunin]

Steps to reproduce

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"
  gem "rails", github: 'rails/rails'
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :users do |t|
    t.references :representer, null: false
    t.timestamps
  end

  create_table :representers do |t|
    t.timestamps
  end

  create_table :posts do |t|
    t.references :representer, null: false
    t.timestamps
  end

  create_table :logs do |t|
    t.timestamps
  end
end

class Log < ActiveRecord::Base; end

class User < ActiveRecord::Base
  belongs_to :representer

  after_commit -> { Log.create! }
end

class Representer < ActiveRecord::Base
  has_one :user

  after_commit -> { Log.create! }
end

class Post < ActiveRecord::Base
  belongs_to :representer
  has_one :user, through: :representer
end

class BugTest < Minitest::Test
  def setup
    # It's false positive when post_id == user_id
    # make user_id different from post_id
    User.create!(representer: Representer.create!)

    @representer = Representer.create!
    @user = User.create!(representer: @representer)
    @post = Post.create!(representer: @representer)
  end

  def test_autosave_for_a_user
    log_count = Log.count
    @post.tap(&:user).save!
    assert_equal(log_count, Log.count)
  end

  def test_autosave_for_a_representer
    log_count = Log.count
    @post.tap(&:representer).save!
    assert_equal(log_count, Log.count)
  end
end

Expected behavior

after_commit should not be called on User on Post saving

Actual behavior

after_commit is called on User despite it was not changed

System configuration

Rails version:
git master 7c63430

Ruby version:
2.5.3p105

The problem happens here:

https://github.com/rails/rails/blob/master/activerecord/lib/active_record/autosave_association.rb#L460

Because User has foreign_key for the representer, and it's representer_id, not post_id. When both IDs don't match, it thinks that the association was changed, and triggers commit for User too