title: Knockout Validation skipping a property
body: <p>I'm using the Knockout Validation plugin (<a href="https://github.com/Knockout-Contrib/Knockout-Validation" rel="nofollow">https://github.com/Knockout-Contrib/Knockout-Validation</a>) to validate my view model but have run </p>
  
  <p>into an issue with properties that have been dynamically added to the view model. They basically get ignored when you call is valid on the </p>
  
  <p>view model.</p>
  
  <pre><code>var myViewModel = ko.validatedObservable({
      property1: ko.observable().extend({ required: true }),
      property2: ko.observable().extend({ max: 10 })
  });
  
  myViewModel()["property3"] = ko.observable().extend({ required: true });
  
  console.log(myViewModel.isValid()); //false
  
  myViewModel().property1('something');
  myViewModel().property2(9);
  
  console.log(myViewModel().property3.isValid()); //false
  console.log(myViewModel.isValid()); //true
  </code></pre>
  
  <p>The above code creates a model with 2 properties and then add a third. Calling isValid on the 3rd property returns false, but when you call </p>
  
  <p>isValid on the model it seems to ignore the 3rd property and returns true.</p>
  
  <p>Any ideas how I can extend a model and the validation still work?</p>