ActiveModel::Dirty is broken in after_commit

[kshnurov]

#45280 broke the expected and documented behaviour by introducing a breaking change in Rails 7.1.
It leads to hard-to-find silent bugs and should be reverted, there's no workaround except disabling it completely with config.active_record.run_commit_callbacks_on_first_saved_instances_in_transaction = true.
The problem it was trying to solve is well-documented and easily dealt with reload.

Steps to reproduce

Oversimplified example:

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "rails"
  # If you want to test against edge Rails replace the previous line with this:
  # gem "rails", github: "rails/rails", branch: "main"

  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 :posts, force: true do |t|
    t.integer :comments_count, null: false, default: 0
  end

  create_table :comments, force: true do |t|
    t.boolean :hidden, null: false, default: false
    t.integer :post_id
  end
end

class Post < ActiveRecord::Base
  has_many :comments

  def update_comments_count!
    self.update!(comments_count: comments.size)
  end
end

class Comment < ActiveRecord::Base
  belongs_to :post
  after_commit :update_posts

  def update_posts
    if saved_change_to_post_id?
      post.update_comments_count!
    end
  end
end

class BugTest < Minitest::Test
  def test_association_stuff
    post_1 = Post.create!
    post_1.comments.create!

    assert_equal 1, post_1.comments_count

    post_2 = Post.create!
    comment = post_2.comments.build
    Post.transaction do
      comment.save!
      comment.update!(hidden: true)
    end
    assert_equal 1, post_1.comments_count
    assert_equal 1, post_2.comments_count
  end
end

Expected behavior

saved_change_to_post_id? == true

Actual behavior

saved_change_to_post_id? == false

System configuration

Rails version: 7.1+
Ruby version: 3.2.3

[jreed-mt]

Is this related to #49898 ?

[palexvs]

@jreed-mt no, it's not. The current issue is reproduced on a much simpler configuration with only one model. You only need Rails 7.1 with default configuration + after_commit => changes is empty even there were changes