rails model callbacks

How can I do something inside the model depending on the whether it's creating or updating a record? I'm sure it's really simple, but I can't seem to figure it out.

These are different styles for updating or creating attachments with paperclip.

class Photo < ActiveRecord::Base

  belongs_to :product

  #after_upate :flag_somthin

  Paperclip.interpolates :product_id  do |attachment, style|
    attachment.instance.product_id
  end

  has_attached_file :data,
    :storage => 's3',
    :s3_credentials => "#{RAILS_ROOT}/config/s3_credentials.yml",
    :bucket => 'leatherarts.com',
    :s3_host_alias =开发者_如何学C> 'leatherarts.com.s3.amazonaws.com',
    :url => ':s3_alias_url',
    :path => "images/products/:product_id/:style/:basename.:extension",
    :styles => lambda { |style| style.instance.choose_styles },
    :default_style => :medium,
    :default_url => 'http://leatherarts.com.s3.amazonaws.com/images/records/m1.png',
    :s3_headers => { 'Expires' => 2.months.from_now.httpdate }

    validates_attachment_presence :data
    validates_attachment_size :data, :less_than => 10.megabytes
    validates_attachment_content_type :data, :content_type => ['image/jpeg','image/gif','image/png']

    def choose_styles
      { :thumb => "60x60#", :small => "200x200>", :medium => "400x400>", :large => "1000x1000>", :backup => "2000x2000>" }, :on => :create
      { :thumb => "60x60#", :small => "200x200>", :medium => "400x400>", :large => "1000x1000>" }, :on => :update
    end

end

Use the new_record? method to return different hashes:

def choose_styles
  defaults = { :thumb => "60x60#", :small => "200x200>", :medium => "400x400>", :large => "1000x1000>" }
  defaults.merge! :backup => "2000x2000>" if new_record?
  defaults
end