update parent in assocation callback

I'd like to update an attribute on the parent using an association
callback. For example

class User < ActiveRecord::Base
  has_many :peeps, :after_add => :update_latest_peep

    def update_latest_peep(peep)
      self.latest_peed_id= peep.id

If I debug and step through the code, I see that @owner (which is the
user in question) actually does have that attribute set however when the
code exists, it doesn't seem to save.