title: knockout : update array of the array in javascript from getJSON
body: <p>here is my code:</p>
  
  <pre><code>var Feed = function (searchTerm, feeditems)
      {
          this.ID = 1;
          this.SearchTerm = searchTerm;
          this.FeedItems = ko.observableArray(feeditems);
      };
  
      var FeedItem = function(userName, message)
      {
          this.UserName = userName;
          this.Message = message;
      };
  
      var viewModel = {
          Feeds: ko.observableArray()
      };
  
      ko.applyBindings(viewModel);
  
  
  
      function getFeedData()
      {
          var url = "/Feed/GetFeedData";
          $.getJSON(url, function (data)
          {
              $.each(data, function ()
              {
                  viewModel.Feeds[0].FeedItems.unshift(new FeedItem(this.UserName, this.Message));
              });
          });
  
          window.setInterval("getFeedData()", TIMEDELAY);
      }
  </code></pre>
  
  <p>I can't figure out how to update the FeedItems of the first Array element of Feeds. 
  basically: viewModel.Feeds[0].FeedItems.unshift does not work... :(  </p>
  
  <p>when i just checking viewModel.Feeds, it gives me all the methods, but with viewModel.Feeds[0] it says it is undefined.</p>
  
  <p>so, how to update the observable array of the first element of the observable array?</p>
  
  <p>thanks!</p>