AngularJS ng-options下拉列表仅针对1个值的Web服务响应失败

Question

我正在使用AngularJS $ http从Web服务填充下拉列表。除非响应只有1个值（例如，汽车装饰件），否则它会正常工作。

HTML：

<select ng-model="carYear" ng-options="item.value for item in cars.years" ng-change="updateDdl('make',carYear.value);" >
            <option value="">Select Year</option>
        </select>
        <select ng-model="carMake" ng-show="cars.makes.length > 0" ng-options="item.value for item in cars.makes" ng-change="updateDdl('model',carMake.value);">
            <option value="">Select Make</option>
        </select>
        <select ng-model="carModel" ng-show="cars.models.length > 0" ng-options="item.value for item in cars.models" ng-change="updateDdl('trim',carModel.value);">
            <option value="">Select Model</option>
        </select>

JS：

angular.module('carData', []).controller('carCtrl', function ($scope, $http) {
        $scope.baseWebAPI = 'https://www.fueleconomy.gov/ws/rest/vehicle/';
        $scope.cars = {};
        $scope.car = {};

        $http({
            method: 'GET',
            url: $scope.baseWebAPI + '/menu/year'
        }).then(function (result) {
            $scope.cars.years = result.data.menuItem;
        });

        $scope.updateDdl = function(t,i){
            switch (t) {
                case "make":
                    $scope.selectedCarYear = i;
                    $scope.webAPI = $scope.baseWebAPI + 'menu/make?year='+i;
                    break;
                case "model":
                    $scope.selectedCarMake = i;
                    $scope.webAPI = $scope.baseWebAPI + 'menu/model?year='+$scope.selectedCarYear+'&make='+i;
                    break;
                case "trim":
                    $scope.selectedCarModel = i;
                    $scope.webAPI = $scope.baseWebAPI +  'menu/options?year='+$scope.selectedCarYear+'&make='+$scope.selectedCarMake+'&model='+i;
                    break;
            }
            $http({
                method: 'GET',
                url: $scope.webAPI
            }).then(function (result) {
                switch (t) {
                    case "make":
                        $scope.cars.makes = result.data.menuItem;
                        break;
                    case "model":
                        $scope.cars.models = result.data.menuItem;
                        break;
                    case "trim":
                        $scope.cars.trims = result.data.menuItem;
                        console.log($scope.cars.trims);
                        break;
                }
            });
        };

“年，年，月，日”字段工作正常，但如果只有一种修整类型且没有任何类型的错误，则修整失败。