Problem With Sorting

Hi

i have 2 models User and Calls with User has many Calls relationship.now i need to show data in cGridview,data is showing correctly but i need to apply sorting.in cGridview i had an attribute 'Called?' Which have value 'yes' or 'no' which represent if Calls Model Have a record corresponding to User.i am using a function in controller to show 'yes' or 'no' in cGridview based on Calls model have an entry or not. How can i implemet sorting on this field?thanks in advance