[Rails] Undefined method updated? for <model object>

Hi,
I have two model say- 
 
ZipCode < ActiveRecord::Base
belongs_to :state_provinces
end

StateProvince < ActiveRecord::Base
has_many :zip_codes
end

if I do @zip_code.update_atribute(:city, "My City")
its giving Undefined method updated? for StateProvince 

if I remove belongs_to specifier in ZipCode class then it work fine.
Any alternate solution??

Thanks
-- 
Regards,
Himanshu

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