How to determine if an instance was created (e.g. not previously persisted) after it has been saved. Very much use case specific.

model.rb

class Model < ActiveRecord::Base
  attr_accessor :was_new_record
  def was_new_record?
    @was_new_record == true ? true : false
  end
end

model_observer.rb

class ModelObserver < ActiveRecord::Observer
  def before_create(model)
    model.was_new_record = true
  end
  def after_commit(model)
    model.do_stuff if model.was_new_record?
  end
end