To quote the docs:
```
If you want to control the display of items in order_by, you can set it to a list or tuple of 2-tuples in the format (field_name, display_name). 
```
so the user is allowed to set them to a list like: ['username', '-username'] -- or did I miss something? (I agree that that's a lazy user, but well, it's not hard to support it ;))