'Look to see if we already have an instance for this particular type
and id.  If so, use it instead of creating a duplicate instance.'

def __new__(cls, id=None, **kwargs):

    if (id is not None):
        instance = cls._instances_by_type_and_id.get((cls, id))
        if instance:
            return instance
    return super(DBObject, cls).__new__(cls, id=id, **kwargs)
