QueryOver select except

Foo.cs

var scheduleDateAlias = default(ScheduleDate);
return session.QueryOver(() => scheduleDateAlias)
    .WithSubquery.WhereNotExists(
        QueryOver.Of<CommunityGrantApplicationChequeRequest>()
        .Where(x => x.ScheduleDate.Id == scheduleDateAlias.Id)
        .Select(x => x.ScheduleDate)
    )
    .OrderBy(scheduleDate => scheduleDate.Rank).Asc
    .List<ScheduleDate>();

The trick is to define the alias in the parent query, then reference it in the subquery.