Demonstrates a bug in ActiveRecord 5.0 where callbacks are not invoked as part of calling `update_attribute`

update_attribute_callbacks_broken.rb

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 "arel", github: "rails/arel"
  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.string :slug
  end
end

class Post < ActiveRecord::Base
  before_save :update_slug

  attr_accessor :new_slug

  def update_slug
    # this operation could be arbitratily complex, what's important
    # is that after the method is called, the object's attributes will
    # have changed as a result of the before_save, so it should be saved
    self.slug = new_slug
  end
end

class BugTest < Minitest::Test
  def test_update_attribute_callbacks_broken
    post = Post.create!(new_slug: "test-slug")

    assert_equal post.slug, "test-slug"

    post.update_attribute(:new_slug, "new-slug")
    post.reload

    assert_equal "new-slug", post.slug
  end
end