Issue using AsNoTracking with EntityFramework

Question

I'm trying to do a deep clone of some entities. The approach mentioned in this article looks good but I run into errors. It suggests using AsNoTracking() to retrieve the entity and then to reinsert it into the context where it will cause an insert as it looks like a new object.

Here's my code:

        Using ctxt As New ProductionDataEntities
            Dim grade = ctxt.Grades.Include(Function(g) g.GradeWidths).AsNoTracking.First
            ctxt.Grades.AddObject(grade)
            ctxt.SaveChanges()
        End Using

But when I run it I get:

An object with the same key already exists in the ObjectStateManager. The existing object is in the Modified state. An object can only be added to the ObjectStateManager again if it is in the added state.

And when I modify grade.Name it's EntityState changes to modified implying that it is being Tracked.

I'm using EF5 Db-First.

Alternatively I've tried cloning by detaching the grade and then reinserting it, this works but the GradeWidths don't get copied. As soon as I call detach the gradewidth count goes from 2 to 0.

Questions:

1. Any idea why AsNoTracking isn't working and what I can do to fix this?
2. Alternatively is there another approach someone can recommend for simple deep cloning?

Thanks.

---- additional info ----

I have 5 1-to-many relationships and eventually I'll want to clone from the top level all the way down. But I'm simplifying it to just look at the lowest level.

• 1 Grade to many GradeWidths

Answer 1

Using ObjectContext

Using ObjectContext you can first attach the parent object and then change object state to added. Also for each child objects, change the object state to added. You even don't need AsNoTracking().

For example for a Category(1)↔(N)Product relation I used this code:

Using db As New SampleSystemEntities
    Dim c = db.Categories.Include(Function(x) x.Products).First
    db.Attach(c)
    db.ObjectStateManager.ChangeObjectState(c, EntityState.Added)
    For Each p As Product In c.Products
        db.ObjectStateManager.ChangeObjectState(p, EntityState.Added)
    Next
    db.SaveChanges()
End Using

Using DbContext

If you use DbContext every thing will work fine using your code:

Using db As New SampleSystemEntities
    Dim c = db.Categories.Include(Function(x) x.Products).AsNoTracking().First
    db.Categories.Add(c)
    db.SaveChanges()
End Using

Answer 2

Without the model is quite hard to understand the exact behaviour (in this case primary Keys are important).
Also, I'm not using EF 5 but EF 6 it should be quite similar.

In your app you are reading an entity with related entities (GradeWidths) from database and you need to add it again to the database.
I suppose that your model is 1 Grade n GradeWidths and you are creating new instances of GradeWidths copied from the original Grade (your model could be n-m and in this case you could use 1 GradeWidth on more Grades but I suppose you are not).

In this case to add new entities you need to reset all the Ids, of the Grade and of the GradeWidths. Then, you have a "clean" clone and you can Add it to the DbSet (Add, not attach).

using (var context = new MyContext(connection))
{
    context.Grades.Add(new Grade()
    {
        GradeWidths = new List<GradeWidth>(new[]
        {
            new GradeWidth() {Width = 10},
            new GradeWidth() {Width = 20},
            new GradeWidth() {Width = 30}
        })
    });
    context.SaveChanges();
}

using (var context = new MyContext(connection))
{
    Grade grade = context.Grades.Include(g => g.GradeWidths).AsNoTracking().First();

    // We need to reset all the ids
    grade.Id = 0;
    foreach (GradeWidth gradeWidth in grade.GradeWidths)
        gradeWidth.Id = 0;


    context.Grades.Add(grade);
    context.SaveChanges();
}

using (var context = new MyContext(connection))
{
    Debug.Assert(context.Grades.Count() == 2);
    Debug.Assert(context.GradeWidths.Count() == 6);
}