javascript - Making dropdown directive in Angular required -

i have written first directive in angular, dropdown, here template :

<select class='form-control' ng-model='vm.scope.fixturetobeupdated.awayteam' ng-options='awayteam.teamname awayteam in vm.scope.awayteams track awayteam.teamid' required>     <option style='display: none' value=''>{{'select' | translate }}</option> </select> 

here defined in controller :

function awayteamsdropdown() {          return {             restrict: "e",             scope: true,             controller: "fixturescontroller",             templateurl: "/views/awayteamsdropdown.html"         };     } 

and here form used in :

<form novalidate id="addupdatefixture" name="addupdatefixture" ng-submit="vm.addupdatefixture()">                          <!--addfixture or updatefixture-->                         <div class="form-group">                             <input type="hidden" ng-model="vm.scope.fixturetobeupdated.fixtureid"/>                         </div>                         <div class="form-group">                             <label for="tournament">{{ 'tournament' | translate }}</label>                             <tournaments-dropdown></tournaments-dropdown>                         </div>                         <div class="form-group">                             <label for="week">{{ 'week' | translate }}</label>                             <weeks-dropdown></weeks-dropdown>                         </div>                         <div class="form-group">                             <label for="awayteamname">{{ 'away_team' | translate }}</label>                             <awayteams-dropdown></awayteams-dropdown>                         </div>                         <div class="form-group">                             <label for="awayteamscore">{{ 'points' | translate }}</label>                             <input id="awayteamscore" type="text" class="form-control" name="awayteamscore"                                    ng-model="vm.scope.fixturetobeupdated.awayteamscore" ng-min="0" ng-max="77" />                             <span ng-show="addupdatefixture.awayteamscore.$dirty && addupdatefixture.awayteamscore.$error.min" class="text-warning">{{ 'score_min' | translate }}</span>                             <span ng-show="addupdatefixture.awayteamscore.$dirty && addupdatefixture.awayteamscore.$error.max" class="text-warning">{{ 'score_max' | translate }}</span>                         </div>                          <div class="form-group">                             <button type="submit" ng-disabled="addupdatefixture.$invalid" class="button bg-darkblue bg-active-darkblue fg-white">{{ 'save' | translate }}</button>                         </div>                     </form> 

but in button in form ng-disabled set false if item not selected dropdown. how make ng-disabled set true if nothing selected in dropdown?

give try this

 <div class="form-group">     <button type="submit" ng-disabled="vm.scope.fixturetobeupdated.awayteam === undefined" class="button bg-darkblue bg-active-darkblue fg-white">{{ 'save' | translate }}</button>  </div> 

or

ng-disabled="!vm.scope.fixturetobeupdated.awayteam"