Find by related objects

I've got two Models (Posts and Categories):

var Post = DS.Model.extend({
  title: DS.attr('string'),
  permalink: DS.attr('string'),
  body: DS.attr('string'),
  category: DS.belongsTo('category')
});

var Category = DS.Model.extend({
  name: DS.attr('string'),
  permalink: DS.attr('string'),
  posts: DS.hasMany('post',{async:true})
});

The relationship is working fine. Now I'd like to request all posts belonging to a certain category (identified by its permalink). What would be the best way to accomplish this?