Angularjs 如何在角材质选择中预加载值？

我想在md select中预加载该值

我有以下代码

<md-select ng-model="country" ng-model-options="{trackBy: '$value.code'}" >
    <md-optgroup label="Country">
        <md-option ng-value="country" ng-repeat="country in countries">
                    {{country.name}}
        </md-option>
    </md-optgroup>
</md-select>

这里我想先加载“美利坚合众国”

---

这目前不起作用。

假设您的服务返回密钥

“usa”

，它会检查下拉数组中是否存在密钥，并将对象分配给

$scope.country

app.controller('myCtrl', function($scope) {
 $scope.service = "usa";
$scope.countries = [{"code":"usa","name":"United States of America"},
  {"code":"ind","name":"India"},
  {"code":"eng","name":"England"},    
  {"code":"aus","name":"Australia"}];

   angular.forEach($scope.countries, function(value) {
        if (value.code === $scope.service ) {
            $scope.country ={code: $scope.service , name: value.name};
            console.log($scope.country);
        }
    });

});

---

我只想发送所选对象的代码。ng value=“country.code”不预加载数据。选择也不起作用。如果我选择任何一个国家，它会选择下拉列表中的最后一个条目。我怎样才能只发送钥匙

app.controller('myCtrl', function($scope) {
 $scope.service = "usa";
$scope.countries = [{"code":"usa","name":"United States of America"},
  {"code":"ind","name":"India"},
  {"code":"eng","name":"England"},    
  {"code":"aus","name":"Australia"}];

   angular.forEach($scope.countries, function(value) {
        if (value.code === $scope.service ) {
            $scope.country ={code: $scope.service , name: value.name};
            console.log($scope.country);
        }
    });

});