trouble dirtying a hasMany associated field with pushObject

gistfile1.js

App.Video = DS.Model.extends({
    responses: DS.hasMany('App.Video')
});

// in a controller
save: function() {
    var v = this.get('model');
    var r = App.Video.find(this.get('responseId'));
    v.get('responses').pushObject(r);
    console.log(v.get('isDirty')); // false
}