from sqlalchemy import *
from sqlalchemy import engine_from_config
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, scoped_session


Base = declarative_base()
Session = scoped_session(sessionmaker())


class A(Base):
    __tablename__ = 'a'
    id = Column(Integer, primary_key=True)
    type = Column(String)

    __mapper_args__ = {'polymorphic_on': type}


class B(A):
    __tablename__ = 'b'
    id = Column(Integer, primary_key=True)
    a_id = Column(Integer, ForeignKey('a.id'))
    subtype = Column(String)
    __mapper_args__ = {'polymorphic_identity': 'B',
                    'polymorphic_on': subtype}



class Bx(A):
    __mapper_args__ = {'polymorphic_identity': 'Bx'}


class By(A):
    __mapper_args__ = {'polymorphic_identity': 'By'}

engine = engine_from_config({'sqlalchemy.url' : 'sqlite:///:memory:'})
Session.configure(bind=engine)
Base.metadata.create_all(engine)

Session.add(B())
Session.add(Bx())
Session.add(By())
print Session.query(A).all()

## Results:

[<__main__.A object at 0x27b5b50>, <__main__.Bx object at 0x27b5d10>, <__main__.By object at 0x27b5c10>]