NHibernate. Field update only on first save

How can I开发者_JS百科 make NHibernate to update any field ONLY on first saving and not to update it on session.Update(obj)?

EDIT: For example, I have an entity A, that has reference to entity B, like:

public class A
{
    // ... some properties
    public virtual B PropB {get; set;}
}

After retrieving the instance of class A I save all its properties instead of PropB into fields on web page (including id and version). After user modified some fields and click 'Save' (herewith I am sure, that he can not edit PropB), I can just restore this object from the web page and save it to the database but I can not restore the linked PropB. So, when I save A instance, it looses link. So, because of PropB can not be modified by any way after first saving, I need a solution to restrict its updating.

There's a mapping attribute that effectively makes a property insert-only: update="false".

However, there are two issues with your question:

1. session.Update does not update an entity, flushing the session does. You only need to call session.Update to attach entities that were not loaded by the session.
2. Why are you modifying a property you don't intend to update in the DB?

I would us DTO's in this case and update the entities within the transaction.

Pseudo-code:

public StoreB(ADto dto)
{
  using (transaction)
  {
    A entity = session.Get<A>(dto.Id);
    entity.PropB = dto.PropB;
    transaction.Commit();
  }
}

public StoreC(ADto dto)
{
  using (transaction)
  {
    A entity = session.Get<A>(dto.Id);
    entity.PropC = dto.PropC;
    transaction.Commit();
  }
}