title: Collection change event on an attribute which is a model itself
body: <p>I have created a Backbone.Collection of (say) ModelA and I am trying to listen to changes to a specific attribute of those models. This is straightforward, ...on('change:attr1'). This fires the event exactly when it needs to. The problem however is when that attribute is another model (ModelB) which has its own attributes. If an attribute of ModelB is changed, the collection won't be able to capture that event (and this is right, the attributes of ModelA never changed, the "attributes of attributes changed"). </p>
  
  <p>An idea is to wire-up listeners from ModelB to ModelA and then to the collection. This could work, but we are now adding a lot of listeners to build this chain of events. I also suspect that it will be a pain to unbind those listeners when we don't need them. </p>
  
  <p>So is there a better way (or alternative) way of achieving this? Any feedback is appreciated.</p>
  
  <p>Thanks</p>