SqlAlchemy AttributeError for Unmapped Subclasses

example_fruits.py

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.associationproxy import association_proxy

metadata = MetaData()

fruitTypesTable = Table(
    'FruitTypes', metadata,
    Column('type_id', Integer, primary_key=True),
    Column('name', String(256))
)

fruitsTable = Table(
    'Fruits', metadata,
    Column('fruit_id', Integer, primary_key=True),
    Column('type_id', Integer, ForeignKey(fruitTypesTable.c.type_id)),
    Column('name', String(256)),
    Column('sweetness', Integer)
)

applesTable = Table(
    'Apples', metadata,
    Column('fruit_id', Integer, ForeignKey(fruitsTable.c.fruit_id), primary_key=True),
    Column('appleyness', Integer)
)

class FruitType(object):
    def __init__(self, *args, **kwds):
        for key in kwds:
            setattr(self, key, kwds[key])
            pass
        return
    def __repr__(self):
        return "{}<{}>".format(self.__class__.__name__, self.__dict__)

class Fruit(object):
    def __init__(self, *args, **kwds):
        for key in kwds:
            setattr(self, key, kwds[key])
            pass
        return
    def __repr__(self):
        return "{}<{}>".format(self.__class__.__name__, self.__dict__)

class Apple(Fruit): pass
class GrannySmith(Fruit): pass
class Rotten(Apple): pass

engine = create_engine('sqlite://')
metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()

mapper(FruitType, fruitTypesTable)

appleType = FruitType(id=1, name='apple')
grannySmithType = FruitType(id=2, name='grannysmith')
session.add(appleType)
session.commit()

mapper(Fruit, fruitsTable,
    polymorphic_on="type_id",
    properties={
        "typeObject": relationship(FruitType)
    })
Fruit.fruitType = association_proxy("typeObject", 'name')

mapper(Apple, applesTable,
    inherits=Fruit,
    polymorphic_identity=1)
mapper(GrannySmith, applesTable,
    inherits=Fruit,
    polymorphic_identity=2)

r = Rotten(appleyness=0) # Error here