为什么Knockout.js数据绑定到checkedValue会覆盖observable

我在使用淘汰映射插件时遇到了一个问题。我有一个ShippingOptions数组，我使用选中的值将其数据绑定到单选按钮，其中选中的属性设置为视图模型上名为ShippingChoice的另一个对象。

我有一个计算的可观察总额，它是我的SubTotal和ShippingChoice的总和。价格

当选择其中一个单选选项时，它会正确地将ShippingChoice设置为所选的运输选项，但会导致ShippingChoice不再是可观察的，并破坏我计算的总

控制台显示错误"类型错误：数字不是函数"，因为我正在访问我的计算中的ShippingChoice（）.Price。一旦设置了值，它似乎会用一个普通对象覆盖我的可观察对象。

我试图使用自定义映射将Shipping选项设置为可观察的，但这没有帮助。

Jsfidle演示：http://jsfiddle.net/on3al/2dv33vor/9/

任何见解或建议都会得到极大的反响。我对这件事太头疼了。

HTML

      <!-- ko foreach: ShippingOptions -->
        <div class="radio">
            <input type="radio" name="optionsRadios" 
                  data-bind="checkedValue: $data ,checked: $root.ShippingChoice" /> 
            <span data-bind="text: Carrier"></span>
            <span data-bind="text: Service"></span>
            <span data-bind="money: Price"></span> 
        </div>
        <!-- /ko -->
    </div>
    <div class="col-xs-4">
         <h5 class="text-right">Subtotal <strong data-bind="money: SubTotal"></strong></h5>
         <h5 class="text-right" data-bind="money: ShippingChoice().Price">Shipping </h5>
         <h4 class="text-right">Total <strong data-bind="money: Total"></strong></h4>
    </div>

Javascript

   var mapping = {
         'ShippingChoice': {
             create: function (options) {
                 return ko.observable(options.data);
             }
         }
     };
     CartViewModel = function (data) {
         var self = this;
         ko.mapping.fromJS(data, mapping, self);
         self.SubTotal = ko.computed(function () {
             var subTotal = 0;
             for (var i = 0; i < self.Items().length; i++) {
                 var item = self.Items()[i];
                 subTotal += item.Quantity() * item.Product.Price();
             }
             return subTotal;
         }, self);
         self.Total = ko.computed(function () {
             return self.SubTotal()+ self.ShippingChoice().Price;
         }, self);
         self.decreaseQty = function (item) {
             var currQty = item.Quantity();
             if (currQty > 0) {
                 item.Quantity(currQty - 1);
             }
             self.updateQty(item);
         };
         self.increaseQty = function (item) {
             var currQty = item.Quantity();
             item.Quantity(currQty + 1);
             self.updateQty(item);
         };
     };
     ko.bindingHandlers.money = {
         update: function (element, valueAccessor, allBindingsAccessor) {
             var value = ko.utils.unwrapObservable(valueAccessor());
             var positions = 2;
             var formattedValue = value.toFixed(positions);
             var finalFormatted = '$' + formattedValue;
             ko.bindingHandlers.text.update(element, function () {
                 return finalFormatted;
             });
         },
         defaultPositions: 2,
     };
     var cartViewModel = new CartViewModel({
         "$id": "1",
             "Id": "540eb73cff622605c4f45b39",
             "Items": [{
             "$id": "2",
                 "Product": {
                 "$id": "3",
                     "Status": "In Stock",
                     "Price": 19.99,
                     "QuantityInStock": 12,
                     "Brand": "Dummy Brand",
                     "Weight": 8.0,
                     "Width": 12.0,
                     "Length": 14.0,
                     "Height": 3.0,
                     "UrlSlug": "dummy-slug",
                     "Title": "Dummy Product",
             },
                 "Quantity": 7,
         }],
             "Country": "CA",
             "PostalCode": null,
             "ShippingOptions": [{
             "$id": "4",
                 "Carrier": "USPS",
                 "Price": 22.1,
                 "Service": "FirstClassPackageInternationalService"
         }, {
             "$id": "5",
                 "Carrier": "USPS",
                 "Price": 32.85,
                 "Service": "PriorityMailInternational"
         }, {
             "$id": "6",
                 "Carrier": "USPS",
                 "Price": 46.96,
                 "Service": "ExpressMailInternational"
         }],
             "ShippingChoice": {
             "$id": "7",
                 "Carrier": "",
                 "Price": 8.0,
                 "Service": ""
         }
     });
     ko.applyBindings(cartViewModel);