Model Computed Property not updating

How come the computed property student.successfulEvaluations does not update unless the page is refreshed?

http://ift.tt/1XIOZhz

App.Student = DS.Model.extend({
  username: DS.attr('string'),
  evaluations: DS.hasMany('evaluation'),
  successfulEvaluations: Ember.computed.filter('evaluations.@each.grade', function(evaluation, index, array){
    return evaluation.get('grade') >= 10;
  })
});

App.Evaluation = DS.Model.extend({
  subject: DS.attr('string'),
  grade:   DS.attr('number'),
  student: DS.belongsTo('student')
});

Ember : 2.2.0
Ember Data : 2.2.1