public function getLastDocument($id_order)
 {
     $order = new ErpOrder($id_order);
     $documents = $order->getDocuments();
     if (count($documents) == 0) {
         return '';
     }
     $document = $documents[0];
     $num_doc = $document->number;
     $num_doc = str_pad($num_doc, 6, '0', STR_PAD_LEFT);
     switch (get_class($documents[0])) {
         case 'OrderInvoice':
             $num_doc = '#' . Configuration::get('PS_INVOICE_PREFIX', $this->context->language->id) . $num_doc;
             break;
         case 'OrderQuotationDetail':
             $num_doc = '#' . Configuration::get('PS_QUOTATION_PREFIX', $this->context->language->id) . $num_doc;
             break;
         default:
             $num_doc = '#' . $num_doc;
             break;
     }
     return $num_doc;
 }