title: Use dynamic url for Backbone Collection
body: <p>Backbone configure url once for all when a Collection is created. Is there a way to change this url later?</p>
  
  <p>The following sample shows 2 POST at <code>/product</code> and 2 <code>POST</code> at <code>/product/id/stock</code>. The last <code>POST</code> won't work, Backbone concatenate the id and try to <code>PUT</code> it, but I don't know why.</p>
  
  <pre><code>products.create({ name: 'American Pastoral', price: 8 });
  products.create({ name: 'The Grapes of Wrath', price: 10 });
  
  products.each(function(product) {
      var id = parseInt(product.get('id'));
      stocks.setId(id);
      stocks.create({ id: id, quantity: 12 });
  }
  </code></pre>
  
  <p>The stock collection:</p>
  
  <pre><code>Backbone.Collection.extend({
      url: function() {
          return this.url;
      },
      parse : function(resp) {
          return resp.stock;
      },
      setProduct: function(id) {
          this.url = '/product/'+id+'/stock';
      }
  });
  </code></pre>
  
  <p>This <strong>won't</strong> work.</p>