title: Checking for any dirty Backbone model data within collection
body: <p>I have a requirement to "nag" a user about unsaved changes when they switch between different Backbone collection models (by clicking on a table row).  I've googled for "check backbone model dirty data" (for instance) and not found anything definitive.</p>
  
  <p>I accomplished this using <a href="http://underscorejs.org/#some" rel="nofollow">underscore's "some"</a> and <a href="http://underscorejs.org/#isEqual" rel="nofollow">isEqual functionality</a>, in a manner such as the following, "some()" being sufficient to determine if there are <em>any</em> un-saved changes (as opposed to what those precise changes might be), in particular because the model attribute is actually an array of objects.</p>
  
  <pre><code>var anyDirty = _.some(myCollection.models, function(model) {
      return !_.isEqual(model.get('nodes'), model.previousAttributes()['nodes]);
  });
  </code></pre>
  
  <p>I am new to Backbone and am wondering if this is an accepted sort of approach for adhoc checking for dirty model data.  Or, does Backbone provide some sort of built in functionality for this purpose, that my initial attempts at googling did not reveal?</p>