Associations_add_association_callbacks.rb

def add_association_callbacks(association_name, options)
  callbacks = %w(before_add after_add before_remove after_remove)
  callbacks.each do |callback_name|
    full_callback_name = "#{callback_name}_for_#{association_name}"
    defined_callbacks = options[callback_name.to_sym]
    if options.has_key?(callback_name.to_sym)
      class_inheritable_reader full_callback_name.to_sym
      write_inheritable_attribute(full_callback_name.to_sym, [defined_callbacks].flatten)
    else
      write_inheritable_attribute(full_callback_name.to_sym, [])
    end
  end
end