Unable to decorate the class inherited from db.Model class of flask-sqlalchmey

I want to decorate the class inherited from db.Model class.
I know it's not recommended in flask-sqlalchemy doc of custom model but i want to do it for other functionalities to work.
Code for the same is in testing_decorater.py file in help-required folder.
what i wanted is access of User class attribute in real time so that i could derive a key from ssid and store that as a password in User model.
If you made it possible then create a new file named flask-crypto/ecrypter.py and put the wrapper class/function code inside it and raise a PR.