Doctrine\ODM\MongoDB\MongoDBException::queryNotIndexed PHP Method

    public static function queryNotIndexed($className, $unindexedFields)
    {
        return new self(sprintf('Cannot execute unindexed queries on %s. Unindexed fields: %s', $className, implode(', ', $unindexedFields)));
    }

 /**
  * Execute the query and returns the results.
  *
  * @return mixed
  */
 public function execute()
 {
     $uow = $this->dm->getUnitOfWork();
     if ($this->isIndexRequired() && !$this->isIndexed()) {
         throw MongoDBException::queryNotIndexed($this->class->name, $this->getUnindexedFields());
     }
     $results = parent::execute();
     $hints = array();
     if ($this->refresh) {
         $hints[self::HINT_REFRESH] = true;
     }
     if ($this->query['slaveOkay'] === true) {
         $hints[self::HINT_SLAVE_OKAY] = true;
     }
     // Unwrap the BaseEagerCursor
     if ($results instanceof BaseEagerCursor) {
         $results = $results->getCursor();
     }
     // Convert the regular mongodb cursor to the odm cursor
     if ($results instanceof BaseCursor) {
         $results = $this->wrapCursor($results, $hints);
     }
     // Wrap odm cursor with EagerCursor if true
     if ($this->query['eagerCursor'] === true) {
         $results = new EagerCursor($results, $this->dm->getUnitOfWork(), $this->class);
     }
     // GeoLocationFindQuery just returns an instance of ArrayIterator so we have to
     // iterator over it and hydrate each object.
     if ($this->query['type'] === self::TYPE_GEO_LOCATION && $this->hydrate) {
         foreach ($results as $key => $result) {
             $document = $result['obj'];
             if ($this->class->distance) {
                 $document[$this->class->distance] = $result['dis'];
             }
             $results[$key] = $uow->getOrCreateDocument($this->class->name, $document, $hints);
         }
         $results->reset();
     }
     if ($this->primers) {
         $documentPersister = $this->dm->getUnitOfWork()->getDocumentPersister($this->class->name);
         foreach ($this->primers as $fieldName => $primer) {
             if ($primer) {
                 $documentPersister->primeCollection($results, $fieldName, $primer, $hints);
             }
         }
     }
     if ($this->hydrate && is_array($results) && isset($results['_id'])) {
         // Convert a single document array to a document object
         $results = $uow->getOrCreateDocument($this->class->name, $results, $hints);
     }
     return $results;
 }