Is there a way to retrieve from a model a list of new relation items added in a recent call to update_attributes

[Mike]

Contrived Model:

class Hobby

  field :name

  belongs_to :person

end

class Person

  has_many :hobbies

  accepts_nested_attributes_for :hobbies

end

Scenario:

I have a form that allows a user to add multiple hobby objects by calling #update_attributes on the retrieved person object to do this. How do I ascertain what new Hobby objects were added by my call to #update_attributes? Is there a helper which identifies these new objects in the person.hobbies Proxy? A use case (not mine, but proves the usefulness) would be to show a flash message saying "Added Hobby X and Hobby Y as Hobbies".