code: @Override
      @JsonIgnore
      public Set<String> getAvailableQueryAttributes(final IPersonAttributeDaoFilter filter) {
          final IUsernameAttributeProvider usernameAttributeProvider = super.getUsernameAttributeProvider();
          return Collections.singleton(usernameAttributeProvider.getUsernameAttribute());
      }
docs: Returns a <code>Set</code> containing only the configured username attribute.