From a question in the mailing list:

How do you do to "rollback" an entity state supposing the following scenario.

public JsonResult Reschedule(string id, DateTime anotherDate)
{
            try
            {
                var dinner = session.Load<Dinner>(id);
                dinner.ChangeDate(anotherDate);
                schedulerService.Schedule(dinner); 

                return Json(new { Message = "Bon apetit!" });
            }
            catch (DinnerConcurrencyException ex) 
            {
                return Json(new { Message = ex.Message });
            }
}

// Base controller
protected void OnActionExecuted(ActionExecutedContext filterContext) 
{
      if(filterContext.Exception != null)
            return;
      session.SaveChanges();
}

The problem that I have is when the schedulerService throws a DinnerConcurrencyException, it is catched at the controller.

After all, the OnActionExecuted will call SaveChanges and persist the dinner with an invalid state.

The next post was:

I have already tried to use Memento Pattern like this:

try
{
    var dinner = session.Load<Dinner>(id);
    dinner.SaveState();
    dinner.ChangeDate(anotherDate);
    schedulerService.Schedule(dinner); 

    return Json(new { Message = "Bon apetit!" });
}
catch (DinnerConcurrencyException ex) 
{
    dinner = dinner.RestoreState();
    return Json(new { Message = ex.Message });
}

But didn't work, beacuse I think Raven has proxied my dinner instance.

And here is the Memento implementation:

[Serializable]
public abstract class Entity
{
    MemoryStream stream = new MemoryStream();

    public void SaveState()
    {
        new BinaryFormatter().Serialize(stream, this);
    }

    public T RestoreState<T>()
    {
        stream.Seek(0, SeekOrigin.Begin);
        object o = new BinaryFormatter().Deserialize(stream);
        stream.Close();

        return (T)o;
    }
}

You might notice that this create a new instance when you call RestoreState, and that has no impact whatsoever on the actual instance that is managed by RavenDB.

The suggested solution?

catch (DinnerConcurrencyException ex) 
{
    SkipCallingSaveChanges = true;
    return Json(new { Message = ex.Message });
}

No need for patterns here.