How to determine if a record is just created or updated in after_save

Yet another option, for those who do have an updated_at timestamp:

if created_at == updated_at
  # it's a newly created record
end

No rails magic here that I know of, you'll have to do it yourself. You could clean this up using a virtual attribute...

In your model class:

def before_save
  @was_a_new_record = new_record?
  return true
end

def after_save
  if @was_a_new_record
    ...
  end
end

I was looking to use this for an after_save callback.

A simpler solution is to use id_changed? (since it won't change on update) or even created_at_changed? if timestamp columns are present.

Update: As @mitsy points out, if this check is needed outside of callbacks then use id_previously_changed?. See docs.