ActiveRecord derived attribute persistence which depends on id value - ruby-on-rails

Question

How do you persist a derived attribute which depends on the value of id in rails? The snippet below seems to work-- Is there a better rails way?
class Model < ActiveRecord::Base
....
def save
super
#derived_attr column exists in DB
self.derived_attr = compute_attr(self.id)
super
end
end

Answer 1

Callbacks are provided so you should never have to override save. The before_save call in the following code is functionally equivalent to all the code in the question.
I've made set_virtual_attr public so that it can be calculated as needed.
class Model < ActiveRecord::Base
...
# this one line is functionally equivalent to the code in the OP.
before_save :set_virtual_attr
attr_reader :virtual_attr
def set_virtual_attr
self.virtual_attr = compute_attr(self.id)
end
private
def compute_attr
...
end
end

Answer 2

I think the more accepted way to do this would be to provide a custom setter for the virtual attribute and then provide an after_create hook to set the value after the record is created.
The following code should do what you want.
class Virt < ActiveRecord::Base
def after_create()
self.virtual_attr = nil # Set it to anything just to invoke the setter
save # Saving will not invoke this callback again as the record exists
# Do NOT try this in after_save or you will get a Stack Overflow
end
def virtual_attr=(value)
write_attribute(:virtual_attr, "ID: #{self.id} #{value}")
end
end
Running this in the console shows the following
v=Virt.new
=> #<Virt id: nil, virtual_attr: nil, created_at: nil, updated_at: nil>
>> v.save
=> true
>> v
=> #<Virt id: 8, virtual_attr: "ID: 8 ", created_at: "2009-12-23 09:25:17",
updated_at: "2009-12-23 09:25:17">
>> Virt.last
=> #<Virt id: 8, virtual_attr: "ID: 8 ", created_at: "2009-12-23 09:25:17",
updated_at: "2009-12-23 09:25:17">