mongoid_test.rb

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'

  gem 'mongoid'
end

require 'yaml'
require 'tempfile'
require 'logger'

uri = 'mongodb://mongo/test'

Tempfile.open(%w(config .yml)) do |f|
  f.write(<<~YAML)
development:
  clients:
    default:
      uri: #{uri}
  YAML
  f.flush

  Mongoid.load!(f.path, 'development')
  Mongoid::Clients.clients.each do |_, client|
    client.logger.level = Logger::ERROR
  end
end

class Foo
  include Mongoid::Document

  embeds_one :bar, cascade_callbacks: true
end

class Bar
  include Mongoid::Document

  embedded_in :foo

  after_save :test_after_save

  def test_after_save
    puts "after_save is called" if $output_after_save_log
  end
end

Foo.delete_all
foo1 = Foo.new(bar: Bar.new)
foo1.save
foo2 = Foo.new(bar: Bar.new)
foo2.save
$output_after_save_log = true

foo1.bar = nil
foo1.save
# => <output nothing>

foo2.update_attributes!(bar: nil)
# => after_save is called