title: How to subscribe to observable in observableArray
body: <p>I'm using KnockoutJS and trying to subscribe to an <code>observable</code> that is in an <code>observableArray</code> that is in an <code>observableArray</code>. So my viewModel looks like this...</p>
  
  <pre><code>function viewModel() {
      // private properties
      var self = this;
  
      // public properties
      self.movies = ko.mapping.fromJS([]);
  
      // subscriptions
      self.movies.UserMovies.Rating.subscribe(function(newValue) {
          console.log(newValue);
      });
  }
  </code></pre>
  
  <p>The <code>movies</code> observableArray would look like this once populated from the mapping plugin...</p>
  
  <pre><code>[{
      Id: 1,
      Title: 'Movie1',
      Year: 2010,
      UserMovies: [{ Id: 11, Rating: 3.5, IsWatched: true }]
  },{
      Id: 2,
      Title: 'Movie2',
      Year: 2010,
      UserMovies: [{ Id: 4, Rating: 4, IsWatched: true }]
  }]
  </code></pre>
  
  <p>I'm trying to set up a subscription to UserMovies.Rating but, from my above viewModel getting the error message </p>
  
  <blockquote>
    <p>TypeError: self.movies.UserMovies is undefined</p>
  </blockquote>
  
  <p><strong>How would I got about setting up a subscription to <code>UserMovies.Rating</code> when it is populated from the mapping plugin?</strong></p>