Handling NHibernate Entity Without Session

NHibernate provides the "session" as a persistence manager interface. The session is used for adding, retrieving and updating any information (viz. entity) in the database. Objects are persisted to the database with the help of the session. A session has many responsibilities, including maintaining the database, transaction and entities context information. In most cases, detached or transient entities can be attached to a session, but sometimes we are unable to determine the entity state, especially if the session it was originally instantiated by has been closed. For these instances, NHibernate sessions provides us with the merge method.

When merging an entity into a session here is what happens:

• If the current session has an entity with the same ID, the changes made to this detached entity are copied to the persistent entity in the session, and it returns the persistent entity.
• If the current session does not have any entity with the same ID, it loads the entity from the database. The changes are copied to the persistent entity, and it returns the persistent entity.
• If the entity does not exist in the database, the current session creates a new persistence entity and copies data from the detached entity.

The merge always returns the same entity that was passed in but the entity now contains all the "merged" changes and is associated with the current session. When the current session transaction is committed, all the changes are saved in the database. The sample code demonstrates how merge can be used. (There is an assert statement to show how the merged entity is different from the passed entity.)

//
//
//
user.FirstName = "Brad";
user.LastName = "Henry";
user.Email = "bradhenry@nexportsolutions.com";

using(var session = SessionFactory.OpenSession())
{
     session.BeginTransaction();
     var mergedUser = session.Merge(user);
     session.Transaction.Commit();
     Assert.False(ReferenceEquals(user,mergedUser));
}
//
//
//

 public User CreateUser()
  {
 var user = new User
  {
   FirstName = "Robert",
   LastName = "Smith",
   Email = "robertsmith@nexportsolutions.com"
  }
 using (var session = SessionFactory.OpenSession())
 {
  session.BeginTransaction();
  session.Save(user);
  session.Transaction.Commit();
  session.Evict(user);
 }
 return user;
}

In conclusion, the session merge enables an entity to be persisted even if that entity is not associated with the current session. This gives the flexibility of processing an entity without an open session. The developer can get an entity with a session, free the session immediately and save the processed entity with another session. Thus, the session is short-lived.

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