Mongoengine删除ReferenceField值

我想删除mongoengine中的ReferenceField值

class User(db.Document):
    name = StringField(required=True)
    last_name = StringField()
    address_ref = ReferenceField(Address)



class Adress(db.Document):
    street = StringField()
    city = StringField()
    state = StringField()

不幸的是,在mongengine文档中。关于unset的信息不多。所以我试着这样做

User.update(id=ObjectId("59ca38c5d1174d081c9bcaca")).unset(address_ref="").first()

但我有个错误

TypeError: update() missing 1 required positional argument: 'self'

我的文档

{
    "_id": {
        "$oid": "59ca38c5d1174d081c9bcaca"
    },
    "name": "Amanda",
    "last_name": "Barow",
    "address_ref": {
        "$oid": "59ca38c9d1174d081c9bcad7"
    }
},
{
    "_id": {
        "$oid": "59ca38c4d1174d081c9bcac9"
    },
    "name": "Larry",
    "last_name": "Denor",
    "address_ref": {
        "$oid": "59c8f34200fc624a0168c4bc"
    }
}

{
    "_id": {
        "$oid": "59ca38c5d1174d081c9bcaca"
    },
    "name": "Amanda",
    "last_name": "Barow",
    "address_ref": ""
},
{
    "_id": {
        "$oid": "59ca38c4d1174d081c9bcac9"
    },
    "name": "Larry",
    "last_name": "Denor",
    "address_ref": {
        "$oid": "59c8f34200fc624a0168c4bc"
    }
}