Can't observe computed property

I've got a controller sorting posts first, then there's a filter applied to them. Sadly when I use {{#each post in filteredPosts}} in my template they don't live update.

How can I achieve the sorting on multiple columns (in different directions), then the filtering AND still have new and deleted posts appear in the list live?

import Ember from 'ember';

export default Ember.ArrayController.extend({
  sortProperties: ['isPublished:asc', 'createdAt:desc'],
  sortedPosts: Ember.computed.sort('model', 'sortProperties'),
  filterPost: '',

  filteredPosts: function() {
    var filterText = this.get('filterPost').toLowerCase()
    return this.get('sortedPosts').filter(function(post){
      return post.get('title').toLowerCase().indexOf(filterText) > -1
    })
  }.observes('sortedPosts').property('filterPost'),

});