title: Backbone.js binding a change event to a collection inside a model
body: <p>I'm pretty new to Backbone so excuse me if this question is a little obvious.
  I am having problems with a collection inside of a model. When the collection changes it doesn't register as a change in the model (and doesn't save).</p>
  
  <p>I have set up my model like so:</p>
  
  <pre><code>var Article = Backbone.Model.extend({
  
     defaults: {
         "emsID" :  $('html').attr('id')
     },
  
     initialize: function() { 
         this.tags = new App.Collections.Tags();
     },
  
     url: '/editorial_dev.php/api/1/article/persist.json'
  });
  </code></pre>
  
  <p>This works fine if I update the tags collection and manually save the model:</p>
  
  <pre><code>this.model.tags.add({p : "a"});
  
  this.model.save();
  </code></pre>
  
  <p>But if the model is not saved the view doesn't notice the change. Can anyone see what I am doing wrong?</p>