Geting added or deleted users from SPContentDatabase

Question

I am trying to get changes from a content database. Below is the code I have used to get the changes, but I get zero results. How to make it work?

 SPChangeQuery query = new SPChangeQuery(false, false);
 query.Group = true;
 query.GroupMembershipAdd = true;
 query.GroupMembershipDelete = true;
 SPUserCollection users = rootWeb.AllUsers;
 SPChangeCollection changes = spSite.ContentDatabase.GetChanges(query);