public function loadProxyModel($relatedTypeKey, $identifier)
    {
        $identifier = $this->convertId($identifier);
        if (true === $this->cache->has($relatedTypeKey, $identifier)) {
            return $this->cache->get($relatedTypeKey, $identifier);
        }

        $metadata = $this->getMetadataForType($relatedTypeKey);
        $model = new Model($metadata, $identifier, $this);
        $this->cache->push($model);
        return $model;
    }