Treeview columns used only for sorting

[Razvan Gavril <razvan gavril gmail com>]

I'm kind of new to gtk programming and have a question about how to do a 
tree view sorting. I have a treeview that displays a list of files. In that 
treeview i have 2 columns: one that display the date when the file was last 
modified and one that display it's size. I want to be able to sort this 
treeview according to this 2 columns. 

The way i do it now is like this: the treemodel has columns for size(hold 
the size in bytes) and date(holds a t_time value) that are hidden in the 
treeview, i use this 2 columns just to do the sorting, but also has 2 more 
columns that display the human readable size and date. I would like to use 
only 2 columns in the treemodel, the visible ones, and still be able to do 
the sorting. Rereading the size and date when sorting is not an option. The 
only idea that i have so far was to make my custom cell renderer for size 
and date but i got stuck and i wanted to know if it isn't any simpler method 
for what i want.