users_groups = Table('users_groups', Base.metadata,
    Column('user_id', Integer, ForeignKey('user.id', ondelete='CASCADE'), primary_key=True),
    Column('group_id', Integer, ForeignKey('group.id', ondelete='CASCADE'), primary_key=True)
    )

class Entity(Base):
    __tablename__ = 'entity'
    id = Column(BigInteger, autoincrement=True, primary_key=True)
    type = Column(Integer, nullable=False)
    __mapper_args__ = {'polymorphic_on': type,
                       'polymorphic_identity': 1}

class Group(Entity):
    __tablename__ = 'group'
    __mapper_args__ = {'polymorphic_identity': 3}

    id = Column(BigInteger, ForeignKey('entity.id', ondelete='CASCADE'), primary_key=True)
    name = Column(Unicode(100))
    url = Column(Unicode(255))
    description = Column(UnicodeText)

    def __init__(self, name):
        self.name = name


friends = Table('friends', Base.metadata,
    Column('user_id', Integer, ForeignKey('user.id', ondelete='CASCADE'), primary_key=True),
    Column('friend_id', Integer, ForeignKey('user.id', ondelete='CASCADE'), primary_key=True)
    )

class User(Entity):
    __tablename__ = 'user'
    __mapper_args__ = {'polymorphic_identity': 2}

    id = Column(BigInteger, ForeignKey('entity.id', ondelete='CASCADE'), primary_key=True)
    firstname = Column(Unicode(50))
    lastname = Column(Unicode(100))
    password = Column(Unicode(50))
    email = Column(String(255))
    email_authed = Column(Boolean)

    groups = relationship('Group', secondary=users_groups, backref='user')

mapper(User, User.__table__, non_primary=True,
           properties={
           'friends': relationship(User,
           secondary=friends,
           primaryjoin=User.id == friends.c.user_id,
           secondaryjoin=friends.c.friend_id == User.id),
           })

user = User()
user.firstname = u'John'
user.lastname = u'Doe'

friend = User()
friend.firstname = u'William the friend'
user.friends = [friend]

self.session.add(user)
self.session.flush()

result = self.session.execute('SELECT * FROM user').fetchall()

print result
result = self.session.execute('SELECT * FROM friends').fetchall()
print result