public function setId($id)
    {
        if ($this->_id !== null && $this->_id != $id) {
            throw new PodException('Cannot update the id of an existing document');
        }

        if (!preg_match('/^\w+\/\w+$/', $id)) {
            throw new PodException('Invalid format for document id');
        }

        @list(, $documentId) = explode('/', $id, 2);
        $this->_key = $documentId;
        $this->_id = (string) $id;
    }