// ----------------------------------
// controllers/InvitationController
// ----------------------------------
    @RequestMapping(value = "/invite/{link}", method = RequestMethod.GET)
    public ModelAndView acceptingInvitation(@PathVariable String link) {
        User user = getLoggedUser();
        Invitation invitation = invitationService.getInvitationByLink(link);
        Permission existingPermission = permissionService.findPermissionById(user.getId(), invitation.getNote().getId());

        if (invitationService.isInvitationValid(link)) {
            Permission newPermission = new Permission();
            newPermission.setId(new UserNoteId(user.getId(), invitation.getNote().getId()));
            newPermission.setPermission(invitation.getPermission());
            ModelAndView redirect = new ModelAndView(new RedirectView("/notes/" + invitation.getNote().getId(), true));

            if (existingPermission == null) {
                permissionService.addPermission(newPermission);
                redirect.addObject("permissionMessage", "Permission to the note has been granted!");
                return redirect;

            } else if (existingPermission.equals(newPermission)) {
                redirect.addObject("permissionMessage", "You already have permission to the note!");
                return redirect;

            } else {
                permissionService.addPermission(newPermission);
                redirect.addObject("permissionMessage", "Permission to the note has been changed!");
                return redirect;
            }

        } else {
            ModelAndView redirect = new ModelAndView("/home/index");
            redirect.addObject("invalidLinkMessage", "Invalid link!");
            return redirect;
        }
    }