How can pre-load the value in Angular material select?

I want to pre-load the value in md-select

I have the following code

<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>

Controller

 $scope.country = "usa"; //I will receive only the key from the server side 
                         //in the response object

$scope.countries = [{"code":"usa","name":"United States of America"},
  {"code":"ind","name":"India"},
  {"code":"eng","name":"England"},    
  {"code":"aus","name":"Australia"}];

Here I want to load "United States of America" initially.

This is currently not working.

Assume your service returns the key "usa", it checks for the existence of key in the drop down array and assign the object to the $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);
        }
    });

});

WORKING DEMO