/**
  * Adds a metadata object into index
  *
  * @param MetadataInterface $metadata
  * @return $this
  */
 public function addMetadata(MetadataInterface $metadata)
 {
     $id = $metadata->getId();
     $metadataItem = array($id => $metadata);
     $this->metadata += $metadataItem;
     foreach ($metadata->getHandles() as $handle) {
         if (!isset($this->metadataByHandle[$handle])) {
             $this->metadataByHandle[$handle] = array();
         }
         $this->metadataByHandle[$handle] += $metadataItem;
     }
     return $this;
 }