public function append(Model $model)
    {
        $result = null;
        // If the list was filtered make sure that value is set on the model.
        if ($this->filter !== null) {
            $result = $this->filter->setFilterValuesOnModel($model);
        }
        $model->save();

        // Make sure the list is refetched.
        $this->invalidate();

        return ($result === null) ? $model : $result;
    }