exceptions.AttributeError: 'property' object has no attribute 'type_map' trying to query from a @classmethod

I'm really not sure what's triggering this, but if I have a fancy object mapped to a table:

t_Customers = Table('customers', metadata,
    Column('customer_id', types.Integer, primary_key=True),
    Column('name', types.Unicode(100), nullable=False),
...
)

class Customer(object):
    primary_key = 'customer_id'
    @classmethod
    def get(cls, pk, *args):
        q = meta.Session.query(cls)
        if args:
            q = q.filter(*args)
        if isinstance(cls.primary_key, str):
            assert not isinstance(pk, (list, tuple)), "The primary key is a single column %s, but the pk provided %s was not a single value" % (cls.primary_key, pk)
            return q.filter(getattr(cls, cls.primary_key) == pk).one()
        if isinstance(cls.primary_key, (list, tuple)):
            assert isinstance(pk, (list, tuple)), "The primary key is multiple columns %s, but the pk provided %s was not a list or tuple" % (cls.primary_key, pk)
            assert len(cls.primary_key) == len(pk), "The primary key is %d columns, but only %d were provided" % (len(cls.primary_key), len(pk))
            #This is just a guess
            for i, col in enumerate(self.primary_key):
                q = q.filter(getattr(cls, col) == pk[i](i))
            return q.one()

orm.mapper(Customer, t_Customers)

Insert a row into the table, and then try to call the get method to retrieve that row:

c = Customer()
c.name = u"Some silly customer name"

Session.save(c)
cid = c.customer_id
Session.flush()

c = Customer.get(cid)

This will sometimes fail with "exceptions.AttributeError: 'property' object has no attribute 'type_map'"

Here's the full stack trace:

File 'pylons_project/controllers/customers.py', line 268 in sendScheduled
  c.customer = model.customers.Customer.get(caff.customer_id)
File 'pylons_project/model/core.py', line 33 in get
  q = meta.Session.query(cls)
File 'site-packages/SQLAlchemy-0.5.0beta1-py2.4.egg/sqlalchemy/orm/scoping.py', line 107 in do
  return getattr(self.registry(), name)(*args, **kwargs)
File 'site-packages/SQLAlchemy-0.5.0beta1-py2.4.egg/sqlalchemy/orm/session.py', line 894 in query
  return self._query_cls(entities, self, **kwargs)
File 'site-packages/SQLAlchemy-0.5.0beta1-py2.4.egg/sqlalchemy/orm/query.py', line 97 in __init__
  self.__setup_aliasizers(self._entities)
File 'site-packages/SQLAlchemy-0.5.0beta1-py2.4.egg/sqlalchemy/orm/query.py', line 111 in __setup_aliasizers
  mapper, selectable, is_aliased_class = _entity_info(entity, ent.entity_name)
File 'site-packages/SQLAlchemy-0.5.0beta1-py2.4.egg/sqlalchemy/orm/util.py', line 398 in _entity_info
  mapper = class_mapper(entity, entity_name, compile)
File 'site-packages/SQLAlchemy-0.5.0beta1-py2.4.egg/sqlalchemy/orm/util.py', line 488 in class_mapper
  mapper = mapper.compile()
File 'site-packages/SQLAlchemy-0.5.0beta1-py2.4.egg/sqlalchemy/orm/mapper.py', line 370 in compile
  mapper.__initialize_properties()
File 'site-packages/SQLAlchemy-0.5.0beta1-py2.4.egg/sqlalchemy/orm/mapper.py', line 391 in __initialize_properties
  prop.init(key, self)
File 'site-packages/SQLAlchemy-0.5.0beta1-py2.4.egg/sqlalchemy/orm/interfaces.py', line 378 in init
  self.do_init()
File 'site-packages/SQLAlchemy-0.5.0beta1-py2.4.egg/sqlalchemy/orm/properties.py', line 210 in do_init
  strategies.DefaultColumnLoader(self)._register_attribute(None, None, False, self.comparator_factory, proxy_property=self.descriptor)
File 'site-packages/SQLAlchemy-0.5.0beta1-py2.4.egg/sqlalchemy/orm/strategies.py', line 37 in _register_attribute
  proxy_property=proxy_property
File 'site-packages/SQLAlchemy-0.5.0beta1-py2.4.egg/sqlalchemy/orm/unitofwork.py', line 88 in register_attribute
  return attributes.register_attribute(class_, key, *args, **kwargs)
File 'site-packages/SQLAlchemy-0.5.0beta1-py2.4.egg/sqlalchemy/orm/attributes.py', line 1407 in register_attribute
  descriptor = proxy_type(key, proxy_property, comparator, parententity)
File 'site-packages/SQLAlchemy-0.5.0beta1-py2.4.egg/sqlalchemy/orm/attributes.py', line 144 in __init__
  self.descriptor = self.user_prop = descriptor
File 'site-packagesupresent/upresent/upresent/model/vertical.py', line 168 in _set_value
  if py_type not in self.type_map:
File 'site-packages/SQLAlchemy-0.5.0beta1-py2.4.egg/sqlalchemy/orm/attributes.py', line 172 in __getattr__
  return getattr(descriptor, attribute)
AttributeError: 'property' object has no attribute 'type_map'

I can put a breakpoint before the call, and run the same line either immediately before or immediately after the traceback. The cls object is set correctly. Doesn't matter if I comment out the extra filtering or not.

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