title: How to perform form validation with fields that are pre-populated via ng-model in Angular
body: <p>Suppose I have a form with required fields.  I am pre-populating these fields via ng-model with data from my controller.  On the submit button for this form, there is an ng-disabled property that checks for $invalid, like this:</p>
  
  <p><code>&lt;button ng-click="next()" ng-disabled="form.is.$invalid"&gt;Next&lt;/button&gt;</code></p>
  
  <p>The problem I am running into is that when the page initially loads, the button is disabled and the validity of the fields are not being checked unless I go back and manually delete each field and re-type their values.</p>
  
  <p>I would like to know how to check for this validity on page load so that if I have some data loading into the fields via ng-model, it will still recognize the required fields as valid.</p>
  
  <p>Thanks.</p>