如何在 angularjs 中验证元素指令

我想在angularjs中验证多选。在多选中应至少选择一个项目。如果没有，则不应启用提交按钮。我正在使用要求来验证表单。我能够通过使用require强制选择名字，但如何强制选择多选。

形式

<form ng-submit='addStudent()' name='studentForm' novalidate="">    
    <div class="col-md-4">
    <div class="col-md-12">
        <label for="Name">First Name *</label>
        <input ng-model='student.first_name' name="name" type="text" placeholder="First Name" required class="form-control">
    </div>
   </div>
   <div class="col-md-4">
    <div class="col-md-12">
        <label for="Name">Last Name </label>
        <input ng-model='student.last_name' name="name" type="text" placeholder="Last Name" class="form-control">
       </div>
   </div>                           
   <multi-selection selectedsubjs="student.selectedSubjects"
            allsubjects="allSubjects"
            left-title="All Subjects"
            right-title="Selected Subjects">
   </multi-selection>                              
   <input ng-disabled="sutudentForm.$invalid" type="submit">
</form>

元素指令

app.directive('multiSelection', function(){
          return {
              restrict: 'E',
              scope: {
                  allsubjects: '=',
                  selectedsubjs: '=',
                  displayAttr: '@',
                  leftTitle: '@',
                  rightTitle: '@'
              },
              templateUrl: "templates/multiSelection.html",
              link: function(scope)   {
                  scope.swapsubject1 = function(item)   {
                      var index = scope.allsubjects.indexOf(item);
                      scope.allsubjects.splice(index, 1);
                      scope.selectedsubjs.push(item);
                  }
                  scope.swapsubject2 = function(item)   {
                      var index = scope.selectedsubjs.indexOf(item);
                      scope.selectedsubjs.splice(index, 1);
                      scope.allsubjects.push(item);
                  }
              }
          };
      });

多选模板

  <tr>
    <td>
        <div class="entBox">
            <switchitem ng-repeat="subj in allsubjects" value="subj.name" ng-click="swapsubject1(item)" ng-model="allSubjects"></switchitem>
        </div>
    </td>
    <td>
        <div class="entBox">
            <switchitem ng-repeat="subj in selectedsubjs" value="subj.name" ng-click="swapsubject2(item)" ng-model="student.selectedSubjects"></switchitem>
        </div>
    </td>
</tr>