private function getKey($key)
    {
        if (!isset($this->cache[$key])) {
            if (!isset($this->mappings[$key])) {
                list($type, $name) = explode('.', $key);
                throw new InvalidArgumentException("No {$type} visitor has been mapped for {$name}");
            }
            $this->cache[$key] = new $this->mappings[$key];
        }

        return $this->cache[$key];
    }