def getSchema(cls):
        """
        return all persistent class attributes
        """
        schema = []
        for name, atr in cls.__attributes__:
            atr = atr.__get__(None, cls)
            if isinstance(atr, SQLAttribute):
                schema.append((name, atr))
        cls.getSchema = staticmethod(lambda schema=schema: schema)
        return schema
return all persistent class attributes