Ng-options不能用于对象

我确信对此有一个很好的解释，但我似乎无法使ng-options与我的模型对象一起工作。基本上，我想让我的选择显示的国家列表。选项值是国家代码，标签是国家名称:

<div ng-app="demoApp">
    <div ng-controller="UserInfoCtrl">
        <h1>User Info</h1>
        <label>Email</label>
        <input type="text" ng-model="user.email" />
        <br />
        <label>Country</label>
        <select ng-model="user.country" ng-options="code as name (code, name) in countriesByCode">
        </select>
        <pre>{{ user | json }}</pre>
    </div>
</div>

var demoApp = angular.module('demoApp', []);
demoApp.controller("UserInfoCtrl", function($scope) {
    $scope.user = { };
    $scope.countriesByCode = {
        'AF' : 'Afghanistan',
        'CA' : 'Canada',
        'RU' : 'Russia'
    };
});