code: public function fromJson(
          $data,
          $className = null,
          $mode = self::UNSERIALIZE_PATCH,
          $document = null
      ) {
          return $this->unserialize(json_dencode($data), $className, $mode, $document);
      }
docs: This will create a document from the supplied json string.
  WARNING: the constructor of the document will not be called.
  
  @param string $data
  @param \Doctrine\ODM\MongoDB\DocumentManager $documentManager
  @param string $className
  @return object