protected function addProductClassField()
    {
        $this->controlAccess('product_class_edit');

        foreach ((array) $this->getSubmitted('field_id', array()) as $field_id) {

            $field = array(
                'field_id' => $field_id,
                'product_class_id' => $this->data_product_class['product_class_id']
            );

            $this->product_class_field->add($field);
        }

        $url = "admin/content/product-class/field/{$this->data_product_class['product_class_id']}";
        $this->redirect($url, $this->text('Product class has been updated'), 'success');
    }