@RequestMapping(value = "/invoice/{id}", method = RequestMethod.GET)
  public Invoice getInvoiceById(@PathVariable int id) {

    Invoice invoice = invoiceService.getInvoiceById(id);
    return addLinks(invoice);
  }
  @RequestMapping(value = "/invoice", method = RequestMethod.GET)
  public InvoiceList getInvoiceByCustomerId(@RequestParam("customerId") int customerId) {

    return addLinks(invoiceService.getInvoiceByCustomerId(customerId), customerId);
  }