Returns a new, unsaved instance of the model class.

def new(cls, **kwargs):
        """Returns a new, unsaved instance of the model class.

        """
        kwargs = cls.preprocess_kwargs_before_new(kwargs)
        if cls.__mapper__.polymorphic_on is not None:
            discriminator_key = cls.__mapper__.polymorphic_on.name
            discriminator_val = kwargs.get(discriminator_key)
            if discriminator_val is not None and discriminator_val in cls.__mapper__.polymorphic_map:
                actual_cls = cls.__mapper__.polymorphic_map[discriminator_val].class_
                return actual_cls(
                    **subdict(
                        actual_cls._preprocess_params(kwargs),
                        actual_cls.all_settable_keys())
                )
        return cls(**subdict(cls._preprocess_params(kwargs), cls.all_settable_keys()))