Question

I started messing with EF 4.0 because I am curious about the POCO possibilities… I wanted to simulate disconnected web environment and wrote the following code to simulate this:

1. Save a test object in the database.
2. Retrieve the test object
3. Dispose of the DataContext associated with the test object I used to retrieve it
4. Update the test object
5. Create a new data context and persist the changes on the test object that are automatically tracked within the DynamicProxy generated against my POCO object.

The problem is that when I call dataContext.SaveChanges in the Test method above, the updates are not applied. The testStore entity shows a status of “Modified” when I check its EntityStateTracker, but it is no longer modified when I view it within the new dataContext’s Stores property. I would have thought that calling the Attach method on the new dataContext would also bring the object’s “Modified” state over, but that appears to not be the case. Is there something I am missing? I am definitely working with self-tracking POCOs using DynamicProxies.

private static void SaveTestStore(string storeName = "TestStore")
{
  using (var context = new DataContext())
  {
    Store newStore = context.Stores.CreateObject();
    newStore.Name = storeName;
    context.Stores.AddObject(newStore);
    context.SaveChanges();
  }
}

private static Store GetStore(string storeName = "TestStore")
{
  using (var context = new DataContext())
  {
    return (from store in context.Stores
            where store.Name == storeName
            select store).SingleOrDefault();
  }
}

[Test]
public void Test_Store_Update_Using_Different_DataContext()
{
  SaveTestStore();
  Store testStore = GetStore();
  testStore.Name = "Updated";      

  using (var dataContext = new DataContext())
  {
    dataContext.Stores.Attach(testStore);
    dataContext.SaveChanges(SaveOptions.DetectChangesBeforeSave);        
  }

  Store updatedStore = GetStore("Updated");
  Assert.IsNotNull(updatedStore);
}

Answer 1

As you stated later, you were using the POCO generator, not the self-tracking entities generator.

I’ve tried it as well, and became quite perplexed. It seems that the proxy classes don’t quite work as expected, and there might be a bug. Then again. none of the examples on MSDN try something like this, and when they reference updates in different tiers of an app (something like we’re doing here) they use self-tracking entities, not POCO proxies.

I’m not sure how these proxies work, but they do seem to store some kind of state (I managed to find the “Modified” state inside the private properties). But it seems that this property is COMPLETELY ignored. When you attach a property to a context, the context adds an entry to the ObjectStateManager, and it stores further state updates in there. At this point if you make a change – it will be registered, and applied.

The problem is that when you .Attach an entity – the Modified state from the proxy is not transferred to the state manager inside the context. Furthermore, if you use context.Refresh() the updates are override, and forgotten! Even if you pass RefreshMode.ClientWins into it. I tried setting the object state’s state property to modified, but it was overridden anyway, and the original settings were restored..

It seems that there’s a bug in the EF right not, and the only way to do this would be to use something like this:

using (var db = new Entities())
{
    var newUser = (from u in db.Users
                    where u.Id == user.Id
                    select u).SingleOrDefault();
    db.Users.ApplyCurrentValues(user);
    db.SaveChanges();
}

One more thing here

Entitity Framework: Change tracking in SOA with POCO approach

It seems that POCO just doesn’t support the approach you’re looking for, and as I expected the self-tracking entities were created to tackle the situation you were testing, while POCO’s proxies track changes only within the context they created.. Or so it seems…