'Returns an ordering value for a particular attribute key.
Unrecognized attributes and OIDs will be sorted lexically at the end.
:return:
An orderable value.'

@classmethod
def preferred_ordinal(cls, attr_name):

    attr_name = cls.map(attr_name)
    if (attr_name in cls.preferred_order):
        ordinal = cls.preferred_order.index(attr_name)
    else:
        ordinal = len(cls.preferred_order)
    return (ordinal, attr_name)
