accepts_nested_attributes_for issue

Hello, I am using accepts_nested_attributes_for for saving the records. let say I have a model class User < AR::Base has_many :books accepts_nested_attributes_for :books end class Book < AR::Base belongs_to :user end Now the params value I am getting is user={:id => 123, books_attributes={:name => “test”}} and in my books controller def create user = @user.update_attributes(params[:user]) @book = ??? end Every thing is working as expected but my requirement is that the book which is saved I need to know the ID. How do I find the books ID. Thanks Abhis