我目前正在使用Flutter和firebase。我正在尝试将一些数据上传到云firestore。到目前为止一切顺利。当我想向字段添加列表时，我遇到了一个问题。如前所述，此字段包含如下定义的值列表：

Map<String, Object> toDocument() {
  Map customerAddress = Map();
  Map orderCheckoutDetails = Map();

  final DateTime now = DateTime.now();
  final DateFormat formatter = DateFormat('dd-MM-yyyy');
  final String formatted = formatter.format(now);

  customerAddress['address'] = this.address;
  customerAddress['city'] = this.city;
  customerAddress['state'] = this.state;
  customerAddress['zipCode'] = this.zipCode;

  orderCheckoutDetails['checkoutOrderDate'] = formatted;
  orderCheckoutDetails['customerAddress'] = customerAddress;
  orderCheckoutDetails['customerName'] = '${this.nome!} ${this.cognome!}';
  orderCheckoutDetails['customerPhone'] = this.numeroTelefono!;
  orderCheckoutDetails['products'] = this.products!.map((product) => product.name).toList();
  orderCheckoutDetails['subtotal'] = this.subtotal!;
  orderCheckoutDetails['deliveryFee'] = this.deliveryFee!;
  orderCheckoutDetails['total'] = this.total!;
  List<Map<dynamic, dynamic>> orderList = [orderCheckoutDetails];

  return {
    'orderCheckoutDetails': orderList,
  };
}

这是它在Firebase上向我显示项目的方式（这是正确的）。enter image description here
这就是我如何将文档上传到Firebase。

@override
 Future<void> addCheckout(Checkout checkout) async {
  print(checkout.email!);

  final docExists = await _checkIfUserCheckoutExists(checkout.email!);
  print(docExists);

  if (!docExists) {
  return _checkoutCollection
      .doc(checkout.email!)
      .set(checkout.toDocument());
  } else {
    await _checkoutCollection.doc(checkout.email!).update({
    "orderCheckoutDetails":
        FieldValue.arrayUnion(checkout.toDocument() as List)
  });
}

}

我想做的是在文档末尾追加一个元素（参数传递的checkout元素），怎么做？