Partial Record Updates with Linq to SQL and MVC - asp.net-mvc

Question

Let's say I have a DB table with columns A and B and I've used the Visual Studio designer to create a Linq objects for this table. All fields are marked NOT NULL.
Now I'm trying to edit this record using typical MVC form editing and model binding, but field B doesn't need to be editable, so I don't include it in the form.
When the post handler binds the object it doesn't populate field B, leaving it null. Now when I save the object I get a DB error saying field B can't be NULL.
The code to save looks something like:
m_DataContext.MyObjects.Attach(myObject);
m_DataContext.Refresh(RefreshMode.KeepCurrentValues, myObject);
m_DataContext.SubmitChanges();
How do I get this to work? Do I need to include field B as a hidden field on the form - I don't really want to do this as it may be updated by other users at the same time and I don't want to stomp over it.
I've found the solution to this problem revolves around getting the entity object associated with the data context before applying the changes. There's a couple of ways of doing this which I've described in separate answers below.

Answer 1

Descend into SQL
This approach ditches LINQ in favour of straight SQL:
public override void SaveMyObject(MyObject o)
{
// Submit
m_DataContext.ExecuteCommand("UPDATE MyObjects SET A={0} WHERE ID={1}", o.ID, o.A);
}
I like this approach the best because of it's simplicity. As much as I like LINQ I just can't justify it's messiness with this problem.

Answer 2

Use a Custom Model Binder
This approach uses a custom model binder to create the entity object and associate with the data context, before the binding takes place.
public class MyObjectBinder : DefaultModelBinder
{
protected override object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType)
{
MyObject a = ((MyObjectController)controllerContext.Controller).Repository.GetMyObjectForUpdate(bindingContext.ValueProvider["ID"].AttemptedValue.ToString());
return a;
}
}
The repository then creates the object and associates it with the data context:
public Object GetMyObjectForUpdate(string id)
{
MyObject o=new MyObject();
o.ID=id;
m_DataContext.Articles.Attach(o);
m_DataContext.Refresh(RefreshMode.KeepCurrentValues);
return o;
}
The action handler needs to be attributed to use the model binder...
[AcceptVerbs(HttpVerbs.Post)]
public ActionResult EditMyObject([ModelBinder(typeof(MyObjectBinder))] MyObject o)
{
if (!ModelState.IsValid)
return View("EditMyObject", a);
Repository.SaveMyObject(a);
return RedirectToAction("Index");
}
and finally SaveMyObject simply calls datacontext.SubmitChanges().
For this to work I also needed to set the update check attributes on all columns to Never (in the dbml file).
Overall, this approach works but is messy.

Answer 3

Use Two Entity Objects
This approach uses two entity objects, one for the model binding and one LINQ:
public override void SaveMyObject(MyObject o)
{
// Create a second object for use with linq and attach to the data context
MyObject o2 = new MyObject();
o2.ID = o.ID;
m_DataContext.Articles.Attach(o2);
m_DataContext.Refresh(RefreshMode.KeepCurrentValues);
// Apply fields edited by the form
o2.A = o.A;
// Submit
m_DataContext.SubmitChanges();
}
This approeach doesn't require any special handling in the controller (ie: no custom model binding) but still requires
the Update Check property to be set to Never in the dbml file.

Answer 4

You could add a timestamp field and check one on the page with the one in the DB (hiding the timestamp field as well). If a user has updated the record, a concurrency error is returned and the page is refreshed, or left the same iwth the users changes.