radio类型的ng-model

radio类型的ng-model，必须是object.object（$scope.email_type.email_type）,否则会是undefined

<div style="text-align: center">
    <div style="width: 15%;display: inline-block">
        <input  name="email" type="radio" ng-model="email_type.email_type" ng-checked="true"  value="type_1"/><span>通知</span>
    </div>
    <div style="width: 15%;display: inline-block">
        <input  name="email" type="radio" ng-model="email_type.email_type"  value="type_2"/><span>提醒</span>
    </div>
    <div style="width: 15%;display: inline-block">
        <input name="email" type="radio" ng-model="email_type.email_type"  value="type_3"/><span>通报</span>
    </div>
    <div style="width: 15%;display: inline-block">
        <input name="email" type="radio" ng-model="email_type.email_type"  value="type_4"/><span>周期通报</span>
    </div>
</div>
$scope.email_type={
    email_type:""
};