Why NHibernate fires an update on firstOrder when saving secondOrder in the code below? I'm using optimistic locking on Order. Is there a way to tell NHibernate to update firstOrder when saving secondOrder only if firstOrder was modified?

// Configure
var cfg = new Configuration();
var configFile = Path.Combine(
  AppDomain.CurrentDomain.BaseDirectory,
  "NHibernate.MySQL.config");
cfg.Configure(configFile);
// Create session factory
var sessionFactory = cfg.BuildSessionFactory();
// Create session
var session = sessionFactory.OpenSession();
// Set session to flush on transaction commit
session.FlushMode = FlushMode.Commit;
// Create first order
var firstOrder = new Order();
var firstOrder_OrderLine = new OrderLine
{
  ProductName = "Bicycle",
  ProductPrice = 120.00M,
  Quantity = 1
};
firstOrder.Add(firstOrder_OrderLine);
// Save first order
using (var tx = session.BeginTransaction())
{
  try
  {
    session.Save(firstOrder);
    tx.Commit();
  }
  catch
  {
    tx.Rollback();
  }
}
// Create second order
var secondOrder = new Order();
var secondOrder_OrderLine = new OrderLine
{
  ProductName = "Hat",
  ProductPrice = 12.00M,
  Quantity = 1
};
secondOrder.Add(secondOrder_OrderLine);
// Save second order
using (var tx = session.BeginTransaction())
{
  try
  {
    session.Save(secondOrder);
    tx.Commit();
  }
  catch
  {
    tx.Rollback();
  }
}
session.Close();
sessionFactory.Close();