public class A 
    {
        public virtual int A { get; set; }
    }

public class A 
    {
        public virtual int A { get; set; }
    }

public class B:A 
    {
        public virtual int B { get; set; }
    }

 public IEnumerable<T> GetAll<T>() where T : Entity<int>
        {
            return CurrentSession.Query<T>().ToList().Where(x => x.GetType() == typeof (T));
}

return CurrentSession.Query<Plot>().Where(x => x is Plot);

PlotMap : JoinedSubclassMapping<Plot>