public function addTag($tag_name)
 {
     $tag_hash = md5($tag_name);
     if (!LernmarktplatzTag::find($tag_hash)) {
         $tag = new LernmarktplatzTag();
         $tag->setId($tag_hash);
         $tag['name'] = $tag_name;
         $tag->store();
     }
     $statement = DBManager::get()->prepare("\n            INSERT IGNORE INTO lernmarktplatz_tags_material\n            SET tag_hash = :tag_hash,\n                material_id = :material_id\n        ");
     return $statement->execute(array('tag_hash' => $tag_hash, 'material_id' => $this->getId()));
 }