无法将文档类的实例转换为其子类的实例--Mongoengine

Question

我正在使用Flask + mongoengine来创建一个Web应用程序。这是我的模型的一部分

class Order(Document):
    placed_on=DateTimeField(required=True)
    order_id = SequenceField()
    status = StringField(default="Request Received")
    cart = ListField(DictField())
    userMobile = StringField(required=True)
    userEmail = StringField(required=True)
    address = ReferenceField(Address)
    sheet_id = IntField(default=0)
    remarks = StringField(default="")

    meta = {'allow_inheritance': True}

class ConfirmedOrder(Order):
    delivery_slot_start = DateTimeField()
    delivery_slot_end = DateTimeField()

    meta = {'allow_inheritance': True}

我有一个Order类的实例作为订单。我现在想将它转换为ConfirmedOrder。这是我用于转换的代码

try:
    order = Order.objects.get(id=order_id)
    cOrder = ConfirmedOrder(**order.to_mongo())
    cOrder.delivery_slot_start = timest
    cOrder.delivery_slot_end = timend
    cOrder.save();
except Exception as e:
    print e

我但是，得到这个错误：

The field '_id' does not exist on the document 'Order.ConfirmedOrder'