protected function attachFields(array &$product)
    {
        if (empty($product)) {
            return $product;
        }

        $product['field'] = $this->product_field->getList($product['product_id']);

        if (empty($product['field']['option'])) {
            return $product;
        }

        foreach ($product['field']['option'] as &$field_values) {
            $field_values = array_unique($field_values);
        }

        return $product;
    }