monkey_patch.rb

# BAD: opening up the original class and defining/redefining methods
class ActiveRecord::Base
  def my_monkey_patch
  end
end

# GOOD: extracting custom logic and *injecting* into core class
module MyCustomLogic
  extend ActiveSupport::Concern

  def my_monkey_patch
  end
end
ActiveRecord::Base.send :include, MyCustomLogic