expire fragment not completing on model save

ok for the ROR experts:: i have a controller that creates another model
(not the same name as the controller) by doing classify.constantize on a
string. then doing a save on that model which implements fragment
caching ... the sweeper method missing gets called (this is normal for
the rails code) but does not complete because @controller is nil in the
ActionController caching.rb module. i want rails to expire the fragment
like it is advertised to do ... any ideas?