NHibernate; Persist child only with uni-directional mapping
===

With uni-directional mapping and Cascade.All on a collection, is it possible to save only a child, by setting a valid FK then do a Save or SaveOrUpdate ? 

If I create a new order, setting a valid Account__id and do a SaveOrUpdate, the Account__id is set to NULL in the database, while the entity still have the Account_id previously set. 

Why? and can I modify perhaps the SaveOrUpdate method to make this work? 

    public class Account
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public IList<Order> Orders { get; set; }
        //...
    }

    public class Order
    {
       public int Id { get; set; }
       public int Account__id { get; set; }
       public decimal Price { get; set; }
       //...
    }

Mappings: 

    public  class AccountMap : ClassMap<Account>
    {
       public AccountMap()
           : base()
       {
           Id(x => x.Id).GeneratedBy.Identity();
           Map(x => x.Name);
           HasMany(x => x.Orders).KeyColumn("Account__id").Cascade.All(); 
       }
    }

Repository: 

        public void SaveOrUpdate(T instance)
        {
            using (ISession session = FnhManager.OpenSession())
            {
                using (ITransaction transaction = session.BeginTransaction())
                {
                    lock (instance)
                    {
                        session.SaveOrUpdate(instance);
                        transaction.Commit();
                    }
                }
            }
        }