public function photos($page = 1, $per_page = 10)
    {
        if (! isset($this->photos["{$page}-{$per_page}"])) {
            $photos = self::get(
                "/categories/{$this->id}/photos",
                ['query' => ['page' => $page, 'per_page' => $per_page]]
            );

            $this->photos["{$page}-{$per_page}"] = [
                'body' => self::getArray($photos->getBody(), __NAMESPACE__.'\\Photo'),
                'headers' => $photos->getHeaders()
            ];
        }

        return new ArrayObject(
            $this->photos["{$page}-{$per_page}"]['body'],
            $this->photos["{$page}-{$per_page}"]['headers']
        );
    }