c# Nhibernate a Different Object with the same Identifier value was already Associated with the Session

Question

I’ve been with that mistake for a while and I can’t find a good solution.

Saving objects works normally, but for changes it returns me this message:

Additional information: a Different Object with the same Identifier value was already Associated with the Session

Class that returns to Session

public class DataAccesLayer {
    private static DataAccesLayer _instance;
    public string ConnectionString { get; set; }

    private DataAccesLayer() {
    }

    private ISessionFactory _sessionFactory;

    private ISessionFactory SessionFactory {
        get {
            return _sessionFactory ?? (_sessionFactory = BuildFactory());
        }
    }

    public static DataAccesLayer Instance {
        get {
            return _instance ?? (_instance = new DataAccesLayer());
        }
    }

    private ISessionFactory BuildFactory() {
        try {
            IPersistenceConfigurer configDB = PostgreSQLConfiguration
                    .PostgreSQL82
                    .ConnectionString(ConnectionString)
                    .ShowSql()
                    .FormatSql()
                    .UseReflectionOptimizer();

            FluentConfiguration FConf = Fluently.Configure()
                .Database(configDB)
                .Mappings(c => c.FluentMappings.AddFromAssemblyOf<System.Retaguarda.Map.UsuarioMap>())
                .ExposeConfiguration(cfg => new SchemaUpdate(cfg).Execute(false, true));

            return FConf.BuildSessionFactory();
        } catch (Exception ex) {
            Console.WriteLine("Erro ao carregar configurações do banco de dados\nDica: Verifique as configurações de conexão\n" + ex.Message + "\n" + ex.InnerException);
            throw ex;
        }
    }

    public ISession OpenSession() {
        return SessionFactory.OpenSession();
    }
}

Save Method

    ISession session = DataAccesLayer.Instance.OpenSession();

    public void Gravar(T entidade) {
        using (ITransaction transacao = session.BeginTransaction()) {
            try {
                session.SaveOrUpdate(entidade);
                transacao.Commit();
                session.Flush();
            } catch (Exception ex) {
                if (!transacao.WasCommitted) {
                    transacao.Rollback();
                }
                Console.WriteLine("Erro ao gravar:\n" + ex.Message + "\n" + ex.InnerException);
                throw ex;
            }
        }
    }

If you use session.Merge(entidade); it normally saves the changes, but of the problem when inserting new items. I thought of a possible solution, but it got a little strange and I don’t intend to use:

    try {
        session.SaveOrUpdate(entidade);
    } catch (Exception) {
        session.Merge(entidade);
    }

Another possibility I tested was running a session.Clear(); before the SaveOrUpdate, but trying to save a "Person" object with "Address" list returns another error:

Additional information: Illegal Attempt to Associate a Collection with two open Sessions

Answer 1

Note that the SaveOrUpdate() should only be used under one of these circumstances:

• The object (entity) is new - that is, the primary key is empty (zeroed, null, etc.) and must be generated; It is equivalent to calling the method Save();
• The object (entity) has a key that already exists in the database, but has not yet been loaded by ISession. It is equivalent to calling the method Update();

The method Merge() does the same procedure above, adding the following behavior:

• If the entity has a valid primary key, read the record to check if it exists in the database (equivalent Get()). If it already exists, it copies the data from the updated entity to the newly loaded entity of the database. If it does not exist, it considers Save();

If, however, an entity is already loaded in the session cache (via method Get() or Lazy-load, for example), any of the above methods will fail.

When an entity is already loaded into ISession, there is no need to call any method to effect the change. This is because Nhibernate is a framework that has change tracking (change-tracking) native.

In your case, the update should be summarized simply like this:

// só isto basta - note que isto irá atualizar (UPDATE) 
// todas as entidades cujas propriedades foram alteradas
session.Flush();