protected function loadMetadata($className)
    {
        if (false === isset($this->loadedMetadata[$className])) {
            if (null === ($class = $this->driver->loadMetadataForClass($className))) {
                // FIXXME Improve this
                throw new \Exception("Can't load metadata for $className");
            }

            $this->completeMetadata($class);
            $this->loadedMetadata[$className] = $class;
        }

        return $this->loadedMetadata[$className];
    }