NHibernate SaveOrUpdate throws NonUniqueObjectException exception - vb.net

Question

I am currently fixing an old Windows Applications and encountered NHibernate error. I've read and tried few things on the net but end up error.
Here is my code for the ISession:
Public ReadOnly Property session() As ISession
Get
If IsNothing(m_session) Then
m_session = Factory.InitConfiguration.OpenSession()
End If
Return m_session
End Get
End Property
Here is my code for the save button:
Try
session.BeginTransaction()
SetParent(x_object)
'session.clear()
session.Flush()
session.SaveOrUpdate(x_object)
session.Transaction.Commit()
compObj.IsNew = False
Return True
Catch ex As Exception
AppServices.ErrorMessage = ex.Message
session.Transaction.Rollback()
Return False
Finally
'TBA
End Try
So the problem start here, I have this date column as DateTime and AttachmentList.
The current code doesn't have any problem until the user key in the year less than 1753. However the code catch the error properly and display the message and when the user continue to fix the year-typo, it'll still catch the error (while at the watch I already get the new value) until the user close the application and reopened it.
However if I uncomment the session.clear(), it will do just fine, the user may fix their typos and continue to save record, but then when the user do the other action lets say attachment, it will get another error. The attachment action as below:
Add Attachment
Click Save button
Add new attachment
Click Save button
New Error.
So please advice me on what need to be done. I've tried merge, I've tried update,save, evict but end up error. I think my problem is how I arrange the session is the main source of the problem.

Answer 1

It looks like the code you have to handle tries to go on using a session having experienced a failed flush or transaction commit.
This is an anti-pattern. From NHibernate reference:
NHibernate use might lead to exceptions, usually HibernateException.
This exception can have a nested inner exception (the root cause), use
the InnerException property to access it.
If the ISession throws an exception you should immediately rollback
the transaction, call ISession.Close() and discard the ISession
instance. Certain methods of ISession will not leave the session in a
consistent state.
...
The following exception handling idiom shows the typical case in NHibernate applications:
using (ISession sess = factory.OpenSession())
using (ITransaction tx = sess.BeginTransaction())
{
// do some work
...
tx.Commit();
}
Or, when manually managing ADO.NET transactions:
ISession sess = factory.openSession();
try
{
// do some work
...
sess.Flush();
currentTransaction.Commit();
}
catch (Exception e)
{
currentTransaction.Rollback();
throw;
}
finally
{
sess.Close();
}
You must ensure the code does not try to go on using the session after an exception.
Furthermore, it looks like the session stay opened while awaiting user interaction: this is not a recommended pattern. A NHibernate session is normally short lived. Opening a session is cheap.
An usual pattern is to open it when starting to process an event from user input and close it before ending the event processing, in order to not leaving it opened while the user is gone taking a coffee.
Now you may have a hard time changing the application session management, especially if the application retains references to entities and expects them to be still bound to an opened session after having waited for user interaction.
If the choice of keeping the session opened between user interaction was done for "leveraging first level cache" (session entity cache), consider activating second level cache instead.