Using a GUID as the ID in a database with ASP.NET MVC

Question

I am learning ASP.NET MVC. I am following one of the basic tutorials on asp.net. Since I do not always follow the tutorials to the letter, I decided to use a GUID as the identity column instead of an integer. Everything was working fine until I got up to the point of adding a new record to the database through the MVC application. When I added the new record, it inserted a blank GUID instead of a generated one. Here is the segment of code-behind that handles the insertion:

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Create([Bind(Exclude = "id")]Movie movieToCreate)
{
    try
    {
        _entities.AddToMovieSet(movieToCreate);
        _entities.SaveChanges();

        return RedirectToAction("Index");
    }
    catch
    {
        return View();
    }
}

The [Bind(Exclude = "id")] line 'ignores' the ID column, since it is autogenerated. In the tutorial, the ID is auto-incremented but I think that is because it is an integer. I tried adding a line to this method:

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Create([Bind(Exclude = "id")]Movie movieToCreate)
{
    try
    {
        movieToCreate.id = Guid.NewGuid();
        _entities.AddToMovieSet(movieToCreate);
        _entities.SaveChanges();

        return RedirectToAction("Index");
    }
    catch
    {
        return View();
    }
}

But the id is still an empty GUID. Can anyone provide me with some information on why this is and perhaps how to resolve it?

Answer 1

You could use a custom ModelBinder. I learned about those over here.

public class MyClassBinder : DefaultModelBinder {
    protected override object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType) {
        var model = (Movie)base.CreateModel(controllerContext, bindingContext, modelType);
        model.id = Guid.NewGuid();
        return model;
    }
}

And your action controller would be:

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult MyAction(Movie movieToCreate) {
    // movie should have a new guid in the id
    _entities.AddToMovieSet(movieToCreate);
    _entities.SaveChanges();
}

And you would need to register the binder in Global.asax:

protected void Application_Start() {
    RegisterRoutes(RouteTable.Routes);
    ModelBinders.Binders.Add(typeof(Movie), new MyClassBinder());
}

Answer 2

The SQL Server EF provider in .NET 3.5 SP1 cannot retrieve server-generated GUIDs, since archaic versions of SQL Server couldn't support that. So you need to generate the GUID client-side, until this limitation is fixed.

You can do this in a model binder, as swilliams suggests, but IMHO binders should only bind data in the request. So I do it on my model tier. But either way will work.

Answer 3

What is the type of ID field in SQL database? Is it uniqueidentifier? If not, change it to uniqueidentifier and make it autogenerated. Check out this article.