Cloud firestore-向对象数组添加对象

{
    address: "123"
    number: "456643634634634"
    postcode: "hshshs"
}

文件定义如下:

userDocument : AngularFirestoreDocument<User>;
this.userDocument = this.afs.collection('users').doc(`${this.userId}`);

addCard(card: Card){
    this.userDocument.update({
        cards : firebase.firestore.FieldValue.arrayUnion(card)
    });
}

但是,在保存文档之前,我会出错,说:

[ts]
Type 'FieldValue' is not assignable to type 'Card[]'.
  Property 'length' is missing in type 'FieldValue'. [2322]
user.model.ts(15, 5): The expected type comes from property 'cards' which is declared here on type 'Partial<User>'

不知道怎么用 arrayUnion