Redmine

I tried to sort users (mainly in watchers list) in a different order than the order of displaing them. While users are to be displayed in the 'firstname lastname' order, I wanted to sort them by their lastname ('lastname firstname'), for example: "John Doe, Ann Smith" instead of the default sorting for this type of display "Ann Smith, John Doe".
I changed the order the ':order' field in /models/user.rb but it seems that it affected nothing - the users are still ordered according to the setting in ':string' field.
What does the ':order' do then?

Environment:
Redmine version 2.4.2.stable
Ruby version 1.9.3-p484
Rails version 3.2.16