private function linkMetaService(string $serviceName, string $key) 
    {
        // make sure the service link is there
        if (!isset($this->metadataService[$serviceName])) {
            $this->metadataService[$serviceName] = [];
        }

        if (!in_array($key, $this->metadataService[$serviceName])) {
            $this->metadataService[$serviceName][] = $key; 
        }
    }