public function save($ISIN, Document $document)
 {
     $data = $document->getArrayCopy();
     $data['ISIN'] = $ISIN;
     if (!$document->id) {
         $this->tableGateway->insert($data);
     } elseif ($this->getDocument($document->id)) {
         $this->tableGateway->update($data, array('id' => $document->id));
     } else {
         throw new \Exception('Document id does not exist');
     }
 }