title: Error: [ngModel:nonassign] Expression is non-assignable
body: <p>Trying to display a columnvalue from a gridcollection based on another value in that same row.
  The user can select/change values in a modal which contains a grid with values. When the modal closes the values are passed back. At that moment I would like to set a value for 'Also known as':</p>
  
  <p>html:</p>
  
  <pre><code> Also known as: &lt;input type="text" `ng-model="displayValue(displayNameData[0].show,displayNameData[0].value)"&gt;`
  </code></pre>
  
  <p>I created a function on scope to select the value only when the 'show' value is true:</p>
  
  <pre><code>$scope.displayValue = function (show, val) {
      if (show) {
          return val;
      }
      else {
          return '';
      }
  }
  </code></pre>
  
  <p>However when I close the modal I get an error:</p>
  
  <pre><code>Error: [ngModel:nonassign] Expression 'displayValue(displayNameData[0].show,displayNameData[0].value)' is non-assignable. 
  </code></pre>
  
  <p>plnkr reference:<a href="http://plnkr.co/edit/UoQHYwAxwdvX0qx7JFVW?p=preview">http://plnkr.co/edit/UoQHYwAxwdvX0qx7JFVW?p=preview</a></p>