public function initialize()
    {
        if (null !== $this->collection) {
            return;
        }

        if (empty($this->dbOptions['database'])) {
            throw new \RuntimeException('The option "database" must be set');
        }
        if (empty($this->dbOptions['collection'])) {
            throw new \RuntimeException('The option "collection" must be set');
        }

        $this->collection = $this
            ->mongo
            ->selectDB($this->dbOptions['database'])
            ->selectCollection($this->dbOptions['collection']);
    }