title: ko.observableArray support for associative arrays
body: <p>Is there a better (built in?) way to mix observableArray and associative arrays?</p>
  
  <pre><code>viewModel = {
      a: ko.observableArray(),
      a_assoc: {},
      add_item: function(i) {
          if (typeof this.a_assoc[i] == 'undefined') {
              this.a.push(i);
              this.a_assoc[i]=1;
          }
      }
  }
  
  viewModel.add_item('bla');
  </code></pre>