title: Angular ngModel not matching any selectbox options
body: <p>I have a problem with selectboxes and an one-way-databinding via [ngModel] in angular:</p>
  
  <p>If my model is storing a value that is not longer existing as an option, the selectbox still takes the value from the model and just shows an "empty" selected option and is valid (See this <a href="https://plnkr.co/edit/FXG6nRJfUMyWNYFOav9A?p=preview" rel="nofollow noreferrer"><code>Plunker example</code></a>). </p>
  
  <p>Is this a bug? Am I doing something wrong? I would expect a different behavior, maybe not taking the value from the model at all or atleast marking the field as invalid. A selectbox shouldn't be valid if the value has no matching options in my opinion.</p>
  
  <p>Is there an easy way to achieve this? I am thinking about adding a custom Validator (that is checking if the value is in a given list or something like that), but i want to avoid this if possible.</p>