output.log

# Running:

saved_changes
{
        "status" => [
        [0] 0,
        [1] 3
    ],
    "updated_at" => [
        [0] 2019-06-07 16:27:36 UTC,
        [1] 2019-06-07 16:27:36 UTC
    ]
}
changes
{
        "status" => [
        [0] 0,
        [1] 3
    ],
    "updated_at" => [
        [0] 2019-06-07 16:27:36 UTC,
        [1] 2019-06-07 16:27:36 UTC
    ]
}
saved_changes
{
        "status" => [
        [0] 0,
        [1] 3
    ],
    "updated_at" => [
        [0] 2019-06-07 16:27:36 UTC,
        [1] 2019-06-07 16:27:36 UTC
    ]
}
changes
{
        "status" => [
        [0] 0,
        [1] 5
    ],
    "updated_at" => [
        [0] 2019-06-07 16:27:36 UTC,
        [1] 2019-06-07 16:27:36 UTC
    ]
}
AFTER MAIN UPDATE
{}
{
            "id" => 1,
        "status" => 5,
    "created_at" => 2019-06-07 16:27:36 UTC,
    "updated_at" => 2019-06-07 16:27:36 UTC
}

scratch_2.rb

# frozen_string_literal: true

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"

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

  # Activate the gem you are reporting the issue against.
  gem "activerecord", "5.1.0"
  gem "sqlite3", '~> 1.3.6'
  gem "awesome_print"
end

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

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

# 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 :status
    t.timestamps null: false
  end
end
ActiveSupport::Deprecation.behavior = ->(_, _) {}

class Post < ActiveRecord::Base
  after_update :inspect_saved_changes

  def inspect_saved_changes
    puts "saved_changes"
    ap saved_changes
    puts "changes"
    ap changes

    if status != 5
      update_attributes(status: 5)
    end
  end
end

class BugTest < Minitest::Test
  def test_saved_changes
    post = Post.create!(status: 0)
    post.update(status: 3)

    puts "AFTER MAIN UPDATE"
    ap post.saved_changes
    ap post.reload.attributes
  end
end