repro

gistfile1.txt

# Note, make sure to run nova-manage db sync first to create a sqlite database
from nova import db
from nova.db.sqlalchemy import models
from nova.db.sqlalchemy import session as sess
from nova import context
from sqlalchemy.orm import joinedload

def instance_get_joined(uuid, session):
    with session.begin():
        val = session.query(models.Instance).filter_by(deleted=False).options(joinedload('instance_type')).filter_by(uuid=uuid).first()
        return val

def instance_get(uuid, session):
    with session.begin():
        val = session.query(models.Instance).filter_by(deleted=False).filter_by(uuid=uuid).first()
        return val

ctxt = context.get_admin_context()
it = db.instance_type_get_by_flavor_id(ctxt, 1)
i = db.instance_create(ctxt, {'instance_type_id': it['id']})
uuid = i['uuid']
session = sess.get_session()
i1 = instance_get_joined(uuid, session)
print 'OK: joined'
print i1['uuid'], i1['instance_type']['name']
print
session = sess.get_session()
i1 = instance_get(uuid, session)
print 'OK: object still in session'
print i1['uuid'], i1['instance_type']['name']
print
session = sess.get_session()
i1 = instance_get_joined(uuid, session)
i1 = instance_get(uuid, session)
session.expunge_all()
print 'OK: object previously joined in same session'
print i1['uuid'], i1['instance_type']['name']
print
session = sess.get_session()
i1 = instance_get(uuid, session)
session.expunge_all()
print 'FAIL: no join and not joined in this session'
print i1['uuid'], i1['instance_type']['name']
print