title: Backbonejs Model.url required?
body: <p>I have an example setup to show my issue: <a href="http://jsfiddle.net/5xo4yzw2/1/" rel="nofollow">http://jsfiddle.net/5xo4yzw2/1/</a></p>
  
  <pre><code>var root = "http://jsonplaceholder.typicode.com"
  
  var Post = Backbone.Model.extend({
      url: function(){
          return this.get('url') || root + "/posts/" + this.get('id')
      }
  });
  
  var Posts = Backbone.Collection.extend({
      model: Post,
      url: root + "/posts"
  });
  
  
  var posts = new Posts();
  posts.fetch();
  console.log("Call to collection fetch works")
  
  var post = new Post({id: 1});
  post.fetch();
  console.log("Call to collection fetch works")
  </code></pre>
  
  <p>This example works, that is, both of the console.logs are executed without an exception being raised by <code>.fetch()</code>. It was my understanding, however, from both <a href="http://backbonejs.org/#Model-url" rel="nofollow">http://backbonejs.org/#Model-url</a> and the book I am reading, that once I specify <code>"/posts"</code> as the collection url, the model url should be automatically resolved to <code>[collection.url]/[id]</code>, though that is not the case. If I do not provide the url function in the model, an exception is raises upon fetching, stating the th url parameter must be provided.</p>
  
  <p>Have i misunderstood something?</p>