Accessing child functions/properties with arranged content in Ember.js

I'm having some trouble figuring out what to do. Basically I have a 'post' controller that has a function 'imageURL'. This function just takes a string (ex image_1.jpg), and prepends the root path to it (ex images/image_1.png). The problem I'm having is in a parent controller, I only want to display the 4 most recent posts. So the problem is that I cant use "imageURL" on the arranged content. Here is the relevant code.

App.HomeController = Ember.ArrayController.extend({
    itemController: 'post',
    sortProperties: ['date'],
    sortAscending: false,

    topPosts: function(){
        var content = this.get('arrangedContent');
        if(content.length >= 4) content = content.splice(0, 3);
        return content;
    }.property('content')
});

App.PostController = Ember.Controller.extend({
    title: Ember.computed.alias('model.title'),
    body: Ember.computed.alias('model.body'),
    postDate: Ember.computed.alias('model.postDate'),
    imageName: Ember.computed.alias('model.imageName'),

    imageUrl: function(){
        var id = this.get('model.id');
        var postImageRoot = "images/posts/" + id + "/";
        var imageName = this.get('model.imageName');
        return postImageRoot + imageName;

    }.property('imageName')
});


App.Post = DS.Model.extend({
    authorId:   DS.attr('number'),
    author:     DS.belongsTo('author'),
    title:      DS.attr('string'),
    body:       DS.attr('string'),
    snippet:    DS.attr('string'),
    postDate:   DS.attr('date'),
    imageName:  DS.attr('string')
});

Any help would be appreciated! :D And I have searched for this for a while and haven't found anything that is specifically like this. Would using an array proxy help? or is there something simple I'm missing?