[sqlalchemy] Problem: TypeError: Incompatible collection type

[Marcos]

Hello!
First, sorry about my english!

I'm having this little problem, and i can't see where is my error. I
have the follow code:

<code>
from sqlalchemy import *
from sqlalchemy.orm import *

class Cuenta(object):
    def __init__(self, codigo, de_titulo, descripcion, padre):
        self.codigo = codigo
        self.de_titulo = de_titulo
        self.descripcion = descripcion
        self.padre = padre

_metadata = MetaData()

cuentas_tabla = Table('cuentas', _metadata,
                      Column('id', Integer, primary_key=True),
                      Column('codigo', String(30), unique=True,
nullable=False),
                      Column('de_titulo', Boolean, nullable=False),
                      Column('descripcion', String(150), unique=True,
nullable=False),
                      Column('cuenta_padre', Integer,
ForeignKey('cuentas.id'), nullable=True)
                      )
mapper(Cuenta, cuentas_tabla, properties={'padre':relation(Cuenta,
backref='hijas')})

engine = create_engine('sqlite:///database.txt')
Session = sessionmaker(bind=engine, autoflush=True,
transactional=False)
_metadata.create_all(bind=engine)
session = Session()
session.begin()
activo = Cuenta(codigo='1',
                de_titulo=True,
                descripcion='Activo',
                padre=None)
fachadaPersistencia.save(activo)
caja = Cuenta(codigo='1.1',
              de_titulo = False,
              descripcion='Caja',
              padre=activo)
fachadaPersistencia.save(caja)
banco = Cuenta(codigo='1.2',
              de_titulo = False,
              descripcion='Banco',
              padre=activo)
fachadaPersistencia.save(banco)
pasivo = Cuenta(codigo='2',
                de_titulo=True,
                descripcion='Pasivo',
                padre=None)
session.save(pasivo)
session.commit()
</code>

And i have the next exception:
<code>
Traceback (most recent call last):
  File "/home/marcos/easyeclipse-python-1.2.2.2/workspace/mine/jContab/
src/jContab/test.py", line 30, in ?
    padre=None)
  File "/usr/lib/python2.4/site-packages/SQLAlchemy-0.4.2-py2.4.egg/
sqlalchemy/orm/attributes.py", line 1126, in init
    oldinit(instance, *args, **kwargs)
  File "/home/marcos/easyeclipse-python-1.2.2.2/workspace/mine/jContab/
src/jContab/test.py", line 9, in __init__
    self.padre = padre
  File "/usr/lib/python2.4/site-packages/SQLAlchemy-0.4.2-py2.4.egg/
sqlalchemy/orm/attributes.py", line 36, in __set__
    self.impl.set(instance._state, value, None)
  File "/usr/lib/python2.4/site-packages/SQLAlchemy-0.4.2-py2.4.egg/
sqlalchemy/orm/attributes.py", line 532, in set
    new_values = list(new_collection.adapt_like_to_iterable(value))
  File "/usr/lib/python2.4/site-packages/SQLAlchemy-0.4.2-py2.4.egg/
sqlalchemy/orm/collections.py", line 509, in adapt_like_to_iterable
    raise TypeError(
TypeError: Incompatible collection type: NoneType is not list-like
</code>

If I quit the backref, i'm still having that error. If anybody can
help me, i will appreciate that.

Thanks,

Marcos Alcazar
Mendoza, Argentina
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