title: Backbone: (Callback) function even not binding to Collection
body: <p>I want updatePlays function to be called as a callback function when an AJAX call is successful. I thought that using underscore bind would let me refer 'this' as the Collection object that I actually want to update, but I'm having trouble here. When I get to the function that should update collection, it thinks that 'this' refers to 'window'. </p>
  
  <p>In this situation, a Backbone Model has Backbone Collection, which are made from another backbone models. </p>
  
  <p>in view: </p>
  
  <pre><code>SomeView: Backbone.View.extend({
    someFunction: function(e) {
      var field = this
      this.picker = new PlayPicker({
        field:field,
        model: new PlaySet({
          plays: new Collections.Plays({}),
          slot: field.model
        })
      })
    }
  }) 
  
  PlayPicker:Backbone.View.extend({
   ...
    refresh: function () {
      this.model.update()
    },
  ....
  </code></pre>
  
  <p>Collection that's part of model PlaySet</p>
  
  <pre><code>Plays:Backbone.Collection.extend({
    model: Play ,
    initialize: function () {
      plays = this
      _.bind(this.updatePlays, plays) // Where I thought I should bind
    },
    updatePlays: function (plays) {
      new_plays = []
  
      var i; 
      for (i = 0; i &lt; plays.length; i++){
        new_plays.push(new Play({
          id: plays[i]["id"],
          text: plays[i]["text"]
        }));
      }
  
      this.reset(new_plays) // Uncaught TypeError: Object [object Window] has no method 'reset' 
    }
  })
  </code></pre>
  
  <p>Model PlaySet</p>
  
  <pre><code>PlaySet: Backbone.Model.extend({
    update: function() {
      this.get('slot').fetchAssociatedPlays(this.get('plays').updatePlays)
    },
  })
  </code></pre>
  
  <p>Model Slot - does the AJAX call</p>
  
  <pre><code>Slot:Backbone.Model.extend({
  
    ...
  
    fetchAssociatedPlays: function(update) {
      thisModel = this
      $.ajax({
        url: thisModel.associatedPlaysURL(),
        success: function (collection) {
          update(collection)
        }
      })
    }})
  </code></pre>
  
  <p>Should this be achievable with underscore bind, and where/how would be the correct way? </p>
  
  <p>Thank you in advance.</p>