title: How to trigger collection's method from a model method with a current model as a parameter
body: <p>I have the following setup:</p>
  
  <pre><code>var Chapter  = Backbone.Model;
  var chapters = new Backbone.Collection;
  
  chapters.add(new Chapter({index: 9, title: "The End"}));
  chapters.add(new Chapter({index: 5, title: "The Middle"}));
  chapters.add(new Chapter({index: 1, title: "The Beginning"}));
  </code></pre>
  
  <p>On request I need to change index of chapters. Is there any way for me to implement method <code>changeIndexes</code> on the <code>Chapters</code> collection with the following syntax:</p>
  
  <pre><code>var Chapters = Backbone.Collection.extend({
    changeIndexes: function(model, bool: increase) {
      // change indexes of the model and sibling models here
    }
  });
  </code></pre>
  
  <p>and methods <code>increase</code> and <code>decrease</code> on the model from collections:</p>
  
  <pre><code>var Chapter = Backbone.Model.extend({
     increase: function() {},
     decrease: function() {}
  );
  </code></pre>
  
  <p>and have method <code>changeIndexes</code> triggered with model and <code>increase=true</code> whenever a <code>modelFromCollection.increse()</code> is triggered, and with <code>increase=false</code> whenever a <code>modelFromCollection.decrease()</code> is triggered?</p>
  
  <p>My first guess is to use custom events propagated within a collection. Is this a way to go or maybe there's a better approach?</p>