Javascript AngularJS表单：根据以前的输入获取第二个选择框的项目

这里的PHP家伙，正在尝试使用AngularJS构建自动更新表单

我的表单如下所示：

<div ng-app="myapp">
 <div ng-controller="ctrl">
  Select Road Name: <select ng-model="roadname" ng-options="x  for x in roadNames" name="forms_roadName"></select>
  Select Bus Stop Name: <select ng-model="busstopname" ng-options="x  for x in busStopNames"  name="forms_BusStopName"></select>
</div>

我希望公共汽车站名称选择框根据所选道路名称自动更新。它没有发生。Chrome控制台不会抛出任何错误，从不满足“if”条件，并且始终执行“else”块。我做错了什么？

使用ng change

 <select ng-change="updateRoad()" ng-model="roadname" ng-options="x  for x in roadNames" name="forms_roadName"></select>

 <select ng-change="updateRoad()" ng-model="roadname" ng-options="x  for x in roadNames" name="forms_roadName"></select>

app.controller('ctrl', function($scope) {
  $scope.roadNames = function_getRoadNames();
    if ($scope.roadname !== null) {  
     $scope.busstopname = function_getBusStopNames($scope.roadname);
    } else { $scope.busstopname = ["Select Roadname first"]; }

   $scope.updateRoad = function() {
        $scope.busstopname = function_getBusStopNames($scope.roadname);
   }
 });