gistfile1.txt

if ([PFUser currentUser]) {
  PFInstallation *currentInstallation = [PFInstallation currentInstallation]; 
  NSString *userId = currentInstallation[@"installationUser"];
  if (![userId isEqualToString:[[PFUser currentUser] objectId]]) {
    currentInstallation[@"installationUser"] = [[PFUser currentUser] objectId];
    [currentInstallation saveEventually];
  }
}