title: Angular Filter not working on ng-options
body: <p>I am new in AngularJs. I have three Select boxes with ng-options. It was working fine before adding filter.</p>
  
  <pre><code>&lt;td style="33%"&gt;
      &lt;select ng-options="car.BaseStation for car in UnUsedCars | orderBy:'BaseStation'" ng-model="selected.BaseStation"&gt;
          &lt;option value=""&gt;All Locations&lt;/option&gt;
      &lt;/select&gt;
  &lt;/td&gt;
  &lt;td style="33%"&gt;
      &lt;select ng-options="car.CarType for car in UnUsedCars | filter: selected.BaseStation | orderBy:'CarType'" ng-model="selected.CarType"&gt;
          &lt;option value=""&gt;All Car Types&lt;/option&gt;
      &lt;/select&gt;
  &lt;/td&gt;
  &lt;td style="33%"&gt;
      &lt;select ng-options="car.Color for car in UnUsedCars | filter: selected.BaseStation | filter: selected.CarType | orderBy:'Color'" ng-model="selected.Color"&gt;
          &lt;option value=""&gt;All Car Colors&lt;/option&gt;
      &lt;/select&gt;
  &lt;/td&gt;
  </code></pre>
  
  <p>The second Select box is expected to act according to the first select box value and third one is expected to work according to the first and second.</p>
  
  <p>The data to these 3 select came from an array of objects 'UnUsedCars'.
  A sample input is as follows:</p>
  
  <pre><code>[{
      "CarType" : "Audi",
      "Color" : "White",
      "BaseStation" : "Canada"
  },
  {
      "CarType" : "BMW",
      "Color" : "White",
      "BaseStation" : "U.S.A"
  },
  {
      "CarType" : "Benz",
      "Color" : "Black",
      "BaseStation" : "Canada"
  }]
  </code></pre>
  
  <p>My JS is as follows</p>
  
  <pre><code>       scope.selected = {
              BaseStation: null,
              CarType: null,
              Color: null
          };
  
          scope.$watch('selected.BaseStation', function() {
              scope.selected.CarType = null;
          });
  
          scope.$watch('selected.BaseStation', 'selected.CarType', function() {
              scope.selected.Color = null;
          });
  </code></pre>
  
  <p>What are the mistakes I made here?</p>