Hi people. I'm trying to update an entity and its related entities as well. For instance, I have a class Car with a property Category and I want to change its Category. So, I have the following methods in the Controller:

public ActionResult Edit(int id)
    {
        var categories = context.Categories.ToList();
        ViewData["categories"] = new SelectList(categories, "Id", "Name");
        var car = context.Cars.Where(c => c.Id == id).First();
        return PartialView("Form", car);
    }

    [AcceptVerbs(HttpVerbs.Post)]
    public ActionResult Edit(Car car)
    {
        var category = context.Categories.Where(c => c.Id == car.Category.Id).First();
        car.Category = category;
        context.UpdateCar(car);
        context.SaveChanges();
        return RedirectToAction("Index");
    }

The UpdateCar method, in ObjectContext class, follows:

public void UpdateCar(Car car)
    {
        var attachedCar = Cars.Where(c => c.Id == car.Id).First();
        ApplyItemUpdates(attachedCar, car);
    }

    private void ApplyItemUpdates(EntityObject originalItem, EntityObject updatedItem)
    {
        try
        {                
            ApplyPropertyChanges(originalItem.EntityKey.EntitySetName, updatedItem);
            ApplyReferencePropertyChanges(updatedItem, originalItem);
        }
        catch (InvalidOperationException ex)
        {
            Console.WriteLine(ex.ToString());
        }
    }        

    public void ApplyReferencePropertyChanges(IEntityWithRelationships newEntity, IEntityWithRelationships oldEntity)
    {
        foreach (var relatedEnd in oldEntity.RelationshipManager.GetAllRelatedEnds())
        {
            var oldRef = relatedEnd as EntityReference;
            if (oldRef != null)
            {
                var newRef = newEntity.RelationshipManager.GetRelatedEnd(oldRef.RelationshipName, oldRef.TargetRoleName) as EntityReference;
                oldRef.EntityKey = newRef.EntityKey;
            }
        }
    }

The problem is that when I set the Category property after the POST in my controller, the entity state changes to Added instead of remaining as Detached.

How can I update one-to-one relationship with Entity Framework and ASP.NET MVC without setting all the properties, one by one like this post?