Entity Framework - Saving Changes to Related Objects in Detached State

Question

I'm using the Northwind database as an example for this post where I am having a problem saving detached entities back to the database using the entity framework.

I have the following two methods to get me the territories and the regions:

    static List<Region> GetRegions()
    {
        using (NorthwindEntities entities = new NorthwindEntities())
        {
            entities.Region.MergeOption = System.Data.Objects.MergeOption.NoTracking;
            return entities.Region.ToList();
        }
    }

    static List<Territories> GetTerritories()
    {
        using (NorthwindEntities entities = new NorthwindEntities())
        {
            entities.Territories.MergeOption = System.Data.Objects.MergeOption.NoTracking;
            return entities.Territories.ToList();
        }
    }

These methods both work fine and get me the collection of objects I require in the detached state.

I also have a static method called SaveEntity, which takes in both an old entity and the currently editted entity, this is as follows:

    static void SaveEntity(EntityObject oldEntity, EntityObject newEntity)
    {
        using (NorthwindEntities entities = new NorthwindEntities())
        {
            entities.Attach(oldEntity);
            entities.ApplyPropertyChanges(newEntity.EntityKey.EntitySetName, newEntity);
            entities.SaveChanges();
        }
    }

This method partially works where the changes to the object are saved down to the database, but any changes to the relationship of related objects are not saved.

I have the following code calling the above methods as my example:

            List<Territories> territories = GetTerritories();
        List<Region> regions = GetRegions();

        Region region = regions.Where(n => n.RegionID == 2).FirstOrDefault();
        Territories oldTerritory = territories.Where(n => n.TerritoryID == "01581").FirstOrDefault();
        Territories newTerritory = ObjectCopier.Clone<Territories>(oldTerritory);

        newTerritory.TerritoryDescription = "Hello World";
        newTerritory.Region = region;

        SaveEntity(oldTerritory, newTerritory);

The change to TerritoryDescription is successfully saved, but the change to Region is not, in the database it still remains as RegionID=1 instead of RegionID=2.

Can anyone provide me with some insight to why ApplyPropertyChanges doesn't propogate changes to related objects?

Also, does anyone know of how I can get around this problem?

Answer 1

Instead of fetching regions and territories seperately, fetch both of them in the same query. Something like (I assume you want to update the entity, do not want to create a new one);

static List<Region> GetTerritoriesWithRegions()
    {
        using (NorthwindEntities entities = new NorthwindEntities())
        {
            entities.Territories.MergeOption = System.Data.Objects.MergeOption.NoTracking;
            return entities.Territories.Include("Region").ToList();
        }
    }

Then update them as the following;

   List<Territories> territoriesWithRegions = GetTerritoriesWithRegions();
        Territories territory = territories.Where(n => n.TerritoryID == "01581").FirstOrDefault();
        territory.TerritoryDescription = "Hello World";
        Region region = territories.Where(p => p.Any(q => q.Region.RegionID == 2)).FirstOrDefault().Region;
        territory.Region = region;

        SaveEntity(territory);

And save them;

static void SaveEntity(EntityObject entity)
    {
        using (NorthwindEntities entities = new NorthwindEntities())
        {
            entities.Attach(entity);
            entities.Context.ObjectStateManager.ChangeObjectState(entity, EntityState.Modified);            
            entities.SaveChanges();
        }
    }

I have coded this on notepad so there may be mistakes; if any please comment so I will update accordingly.