code: @Override
  	public void validate(IValidatable<String> validatable) {
  		String value = validatable.getValue();
  		String path = StorageUtil.createPath(parentPath, value);		
  		if (storageService.entityExists(path)) {			
  			ValidationError error = new ValidationError();
  			String message = String.format(errorMessage, value);
  			error.setMessage(message);
  			validatable.error(error);
  		}
  	}
docs: TODO wicket-6