Class variables in models

I am trying to do the following:

- initialise a class variable for a model class (@@sub_types = [])
- store an array of values into this class variable during the initial
load of a "model" class
- use this class variable and the populated array within a class
method in the same model when servicing subsequent HTTP requests to my

My tests show that the class variable is initialised/populated when
the model class is first loaded (triggered by a model method call in
class ApplicationController) BUT does not persist for use when
processing requests.

fyi, I have configured development.rb to cache classes.

Why is the class variable not persisting across requests?