nonglobal sessions, no dbsession attached to request

models.py

import transaction

from sqlalchemy.ext.declarative import declarative_base

from sqlalchemy.exc import IntegrityError
from sqlalchemy.orm.session import object_session

from sqlalchemy import Integer
from sqlalchemy import Unicode
from sqlalchemy import Column

Base = declarative_base()

class MyModel(Base):
    __tablename__ = 'models'
    id = Column(Integer, primary_key=True)
    name = Column(Unicode(255), unique=True)
    value = Column(Integer)

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

    @property
    def _session(self):
        return object_session(self)

class MyRoot(object):
    __name__ = None
    __parent__ = None

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

    def __getitem__(self, key):
        session = self.session
        try:
            id = int(key)
        except (ValueError, TypeError):
            raise KeyError(key)

        item = session.query(MyModel).get(id)
        if item is None:
            raise KeyError(key)

        item.__parent__ = self
        item.__name__ = key
        return item

    def get(self, key, default=None):
        try:
            item = self.__getitem__(key)
        except KeyError:
            item = default
        return item

    def __iter__(self):
        session= self.session
        query = session.query(MyModel)
        return iter(query)

def root_factory(request):
    session = request.registry.settings['db.sessionmaker']()
    root = MyRoot(session)
    return root

def populate(session):
    try:
        model = MyModel(name=u'test name', value=55)
        session.add(model)
        session.flush()
        transaction.commit()
    except IntegrityError:
        session.rollback()