How can i search using selectbox in angularjs

I have thing working correctly in angular

   Any: <input ng-model="search.$"> 
   Student Name only <input ng-model="search.number">
   student Region only <input ng-model="search.region">

and this

<li ng:repeat="students in student | filter:search"  >

But i want to make those three search box into one search box and select box like this

  Any: <input ng-model="search.$"> 

<select>
  <option value="number">Student Number</option>
  <option value="region">Student Region</option>

</select> 

Also i would like to perform search when user click on search button

How can i do that

You could set your search value when user hits the button. live sample: http://plnkr.co/edit/jTRmBH.

template:

 <label>
 Search Value: <input ng-model="filter">*emphasized text*
 </label><br>
 <label>
 Search Key:
 <select ng-model="key">
   <option value="$">Any</option>
   <option value="number">Student Number</option>
   <option value="region">Student Region</option>
 </select> 
 </label>
 <button ng-click="setfilter()">search</button>

javascript:

$scope.setfilter = function() {
  $scope.search = {};
  $scope.search[ $scope.key ] = $scope.filter;
  console.log( $scope.search );
};