title: KnockoutJS Validation - nested ViewModel as validatedobservable
body: <p>I have a KnockoutJS viewmodel that acts as a "master" viewmodel for several modular viewmodels. These modular vm's utilize knockout validated properties for their own form validations. I would like to make these viewmodels <code>validatedObservable</code> in the master viewmodel, so I can simply check each if <code>isValid</code> is true:</p>
  
  <pre><code>function MasterViewModel() {
      var self = this;
  
      self.addUserViewModel = ko.validatedObservable(new AddUserViewModel());
  }
  
  function AddUserViewModel() {
      var self = this;
      self.username = ko.observable().extend({ required: true });
  
      self.addUser = = function () {
          if(self.isValid()) {
              // Perform add user logic here
          }
      }.bind(self);
  }
  </code></pre>
  
  <p>However, the <code>isValid</code> method does not exist for the child viewmodel when I attempt to call it. Is there an issue with adding a <code>validatedObservable</code> property that is a function?</p>