Angular get the value the selected for the model.

I got a problem with ng-model and ng-selected. At the beginning the ng-model is null, and the correct value is selected. So if you submit the form you submit a form like this { resolution : undefnied, desiredFps : 30 } and it's incorrect. So I want that the model of the take the value of the selected value designed with ng-selected. What is the correct way of updating the selected value and updating the model with the value as well?

Thanks for you help !

    <form novalidate name="preferencesForm" ng-submit="submitPreferencesForm(BEPreferencesForm)" ng-controller="UserPreferencesFormController">

        <label for="resolution">Choose the resolution : </label>
        <br/>
        <select name="resolution" id="resolution"  ng-model="BEPreferencesForm.resolutionId" ng-change="buttonDisabled = False">
            <option value="1" ng-selected="user.screenWidth ==  800 && user.screenHeight == 600">800x600</option>
            <option value="2" ng-selected="user.screenWidth ==  1024 && user.screenHeight == 768">1024x768</option>
            <option value="3" ng-selected="user.screenWidth ==  1280 && user.screenHeight == 720">1280x720</option>
            <option value="4" ng-selected="user.screenWidth ==  1280 && user.screenHeight == 768">1280x768</option>
            <option value="5" ng-selected="user.screenWidth ==  1360 && user.screenHeight == 768">1360x768</option>
            <option value="6" ng-selected="user.screenWidth ==  1600 && user.screenHeight == 900">1600x900</option>
            <option value="7" ng-selected="user.screenWidth ==  1768 && user.screenHeight == 992">1768x992</option>
            <option value="8" ng-selected="user.screenWidth ==  1920 && user.screenHeight == 1080">1920x1080</option>
        </select>