How to get parent route models in the child route model() method.

gistfile1.js

App.Router.map(function() {
  this.resource('posts', function() {
    this.resource('post', { path: '/:post_id' }, function() {
      this.resource('permissions', function() { ... });
    });
  });
});

App.PermissionsRoute = Ember.Route.extend({
  model: function() {
    // need parent route model
    return App.Permission.find({ post_id: parent.model.id });
  }
});

// Currently I access the parent routes controller for its model like:

App.PermissionsRoute = Ember.Route.extend({
  model: function() {
    var post = this.controllerFor('post').get('model');
    return App.Permission.find({ post_id: post.get('id') });
  }
});

// But, this doesn't work when the permissions route is entered via url