How can I set a ringtone for an individual contact on Android ?

I have found a way to set the default ringtone that applies to all contacts without an individual ringtone. But that is not what i'm trying to accomplish.

I want the application to have a button "Apply ringtone to contact". When i click, I start an activityForResult displaying a list of all contacts on the phone. When a contact is selected, the contact activity closes and returns with a uri to the contact. Now all the app needs to do is to apply the selected ringtone to that spesific contact.

The code for displaying and selecting contacts by an activity is already implemented and seems to work on with the app. Its only the last part left, and I have no clue about how to solve this.

Any help would be usefull.