title: Why do I need to "reopenClass" to set the url for an ember-data model?
body: <p>I found that if I try to include the url in the original definition of an ember-data model it blows up in my REST adapter but if I simply "reopenClass" it's fine.</p>
  
  <p>What is the technical reason behind this? (below is the working example)</p>
  
  <pre><code>CodeCamp.Speaker = DS.Model.extend({
      id: DS.attr('number'),
      name: DS.attr('string'),
      session: DS.belongsTo('CodeCamp.Session')
  });
  
  CodeCamp.Speaker.reopenClass({
      url: 'sessions/%@/speakers'
  });
  </code></pre>