sebastian gomez: March 2011

I haven't been working on anything outside Genexus, Deklarit and Genexus Server for a while, so when my friend Mateo asked to help him on a new project for a client of his, based on brand new Microsoft technologies, I was saying yes before the end of the sentence.

This blog post and probably some more to come will be related to our experience with Entity Framework 4, RIA Services 1 and Silverlight 4.

But in this particular case I wanted to blog about a problem we had with Compositions. Compositions are very useful when you have an Association where a Parent entity needs to have its children all the time.

The Update method of parent entities is a bit different than regular entities. I took the patter from this article (Compositional Hierarchies) but things didn’t work as expected. I have to add that “m generating POCO entities and using the Unit Of Work pattern so it wasn’t easy just copy and paste the code for the article. I posted a few questions to the RIA services forum and found out that that pattern exposed a bug :(

Fortunately I was redirected this post from Brett Samblament which described the new pattern to follow to write a fully functional UpdateParent method. But again, I can’t just copy and paste, so here’s the code I wrote for it. Also, by using generic, I’m able to call the exact pattern for every composition in my model, cool uh?!

public void UpdateParent(Parent parent)
{
    EntityHelper.UpdateParentEntity(parent, ObjectContext.Parents, ChangeSet, ObjectContext);

    foreach (Child child in ChangeSet.GetAssociatedChanges(parent, o => o.Children))
        EntityHelper.UpdateChildEntity(child, ObjectContext.Children, ChangeSet, ObjectContext);

}

And my EntityHelper class has the following methods:

public static void UpdateParentEntity<T>(T entity, IRepository<T> repository, ChangeSet chgSet, IUnitOfWork oc)
    where T : class
{
    try
    {
        ObjectContext ctx = oc as ObjectContext;
        repository.ObjectSet.AddObject(entity);

        T originalEntity = chgSet.GetOriginal<T>(entity);

        if (originalEntity == null)
            ctx.ObjectStateManager.ChangeObjectState(entity, EntityState.Unchanged);
        else
            repository.ObjectSet.AttachAsModified(entity, originalEntity);
    }
    catch (Exception ex)
    {
        TraceManager.Error(string.Format("An error occurred updating a {0}", typeof(T)), ex);
        throw;
    }
}

public static void UpdateChildEntity<T>(T entity, IRepository<T> repository, ChangeSet chgSet, IUnitOfWork oc)
    where T : class
{
    try
    {
        ObjectContext ctx = oc as ObjectContext;
        ChangeOperation change = chgSet.GetChangeOperation(entity);
        
        switch (change)
        {
            case ChangeOperation.Delete:
                if (GetEntityState(entity, ctx) == EntityState.Detached)
                    repository.ObjectSet.Attach(entity);
                ctx.DeleteObject(entity);
                break;
            case ChangeOperation.Insert:
                // do nothing
                break;
            case ChangeOperation.None:
                ctx.ObjectStateManager.ChangeObjectState(entity, EntityState.Unchanged);
                break;
            case ChangeOperation.Update:
                T original = chgSet.GetOriginal<T>(entity);
                if (original == null) { throw new Exception("Update with no original value found"); }
                if (GetEntityState(entity, ctx) == EntityState.Detached)
                    repository.ObjectSet.Attach(entity);
                repository.ObjectSet.AttachAsModified(entity, original);
                break;
            default:
                break;
        }
    }
    catch (Exception ex)
    {
        TraceManager.Error(string.Format("An error occurred updating a {0}", typeof(T)), ex);
        throw;
    }
}

public static EntityState GetEntityState(object entity, ObjectContext ctx)
{
    System.Data.Objects.ObjectStateEntry ose;
    if (ctx.ObjectStateManager.TryGetObjectStateEntry(entity, out ose))
        return ose.State;
    else
        return EntityState.Detached;
}