STI class_name returns the parent class name

Is this a bug or intended action - or am I missing something.

If I have
class Thing < ActiveRecord::Base
and
class ZThing < Thing

and I do

z=ZThing.create

Then z.class
returns ZThing(id: ....etc
as I would expect

but z.class.class_name
returns Thing

which is a problem when I want to test for my STI class. Or should I
check for STI class name in a different way?

Thanks
Tonypm.