polymorphic_trick.py

from sqlalchemy import MetaData, Column, ForeignKey, types, create_engine, orm
from sqlalchemy.ext.declarative import declarative_base, declared_attr


engine = create_engine('sqlite://')
sm = orm.sessionmaker(bind=engine)
metadata = MetaData()
metadata.bind = engine

Base = declarative_base(metadata=metadata)


class PolymorphicMixin(object):
    discriminator = Column(types.String(50))

    @declared_attr
    def __mapper_args__(cls):
        ret = {'polymorphic_identity': cls.__name__}
        if Base in cls.__bases__:
            ret['polymorphic_on'] = cls.discriminator
        return ret


class Employee(Base, PolymorphicMixin):
    __tablename__ = 'employees'
    id = Column(types.Integer, primary_key=True)
    name = Column(types.String(50))


class Developer(Employee):
    language = Column(types.String(50), default='python')


class Hacker(Developer):
    editor = Column(types.String(50), default='vim')


#### use it ####
metadata.create_all()
sess = orm.scoped_session(sm)

dev = Developer(name='joe')
sess.add(dev)

hacker = Hacker(name='sam')
sess.add(hacker)

sess.commit()

print "**** employees ****"
print sess.query(Employee).all()
print "**** hackers ****"
print sess.query(Hacker).all()