public class Device 
    {
        public virtual int UDId { get; set; }
        public virtual int Id { get; protected set; }        
    }

public class DeviceMap : ClassMap<Device>
    {
        public DeviceMap()
        {
            // works fine : 
            Id(x => x.Id);

            // schema is created but exception is thrown during the save part
            //Id(x => x.Id).GeneratedBy.Assigned();

            //schema generation fails
            //Map(x => x.Id).Generated.Insert();

            Map(x => x.UDId);
        }
    }

public class DeviceMap : ClassMap<Device>
    {
        public DeviceMap()
        {
            // schema is created
            Id(x => x.Id).GeneratedBy.Native(); //Assigned();
            
            Map(x => x.UDId);
        }
    }

using (var currentSession = session.OpenSession())
            {
                using (var transaction = currentSession.BeginTransaction())
                {
                    try
                    {
                        currentSession.SaveOrUpdate(entity);
                        transaction.Commit();
                    }
                    catch (Exception)
                    {
                        transaction.Rollback();
                        throw;
                    }
                }    
            }

using (var currentSession = session.OpenSession())
            {
                using (var transaction = currentSession.BeginTransaction())
                {
                    try
                    {
                        if (isNew)
                            {
                                currentSession.Save(entity);
                            }
                            else
                            {
                                currentSession.Update(entity);
                            }
                        transaction.Commit();
                    }
                    catch (Exception)
                    {
                        transaction.Rollback();
                        throw;
                    }
                }    
            }

Cannot insert the value NULL into column 'Id', table 'test.dbo.Device'; column does not allow nulls. INSERT fails.
The statement has been terminated.