Entity Framework with ASP.NET MVC

Question

How can I use strongly-typed Controllers with EntityObjects?

My failures...

First I tried this:

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Edit(Guid id, Department Model)
{
    db.SaveChanges();
    return RedirectToAction("Index");
}

This failed to actually save any changes to the database. So, I tried to attach the model to my ObjectContext:

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Edit(Guid id, Department Model)
{
    db.Attach(Model);
    db.SaveChanges();
    return RedirectToAction("Index");
}

This failed because "an object with a null EntityKey value cannot be attached to an object context." So, I tried to assign the EntityKey:

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Edit(Guid id, Department Model)
{
    Model.EntityKey = (from Department d in db.Department
                       where d.Id == id
                       select d).FirstOrDefault().EntityKey;
    db.Attach(Model);
    db.SaveChanges();
    return RedirectToAction("Index");
}

This failed because "an object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key."

How is this supposed to work?

Answer 1

Try db.AttachTo() instead of db.Attach()

As described here

"Call AttachTo on ObjectContext to attach the object to a specific entity set in the object context. Also do this if the object has a null (Nothing in Visual Basic) EntityKey value."

Edit: actually, you may need to call ApplyPropertyChanges() in this case.

Edit 2: You can build an entity key for your object with code like the following:

public EntityKey GetEntityKey(string entitySetName, object keyValue)
{
  IEnumerable<KeyValuePair<string, object>> entityKeyValues =
    new[]
    {
      new KeyValuePair<string, object>("Id", keyValue)
    };

  var key = new EntityKey(string.Concat("YOUR_OBJECT_CONTEXT_NAME.", entitySetName), entityKeyValues);
  return key;
}

And then you'd call this with something like this for entity keys based on a single database column called 'Id':

var entityKey = GetEntityKey("Customer", 23);

I use a similar method in my own EF code except with a Generic type argument to avoid having to use a string for the entity class name.

Answer 2

The thing is, what you have as your input is not a complete model object with changed properites - it is just a "start" of new one, with all the properties that you want to overwrite on an already existing entity. You don't post any of the ID properties,

This looks a little verbose, but it's the best I've found, so it is how I do it in my projects (ajusted to your class names, with whatever was missing made up...):

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Edit(Guid id, Department Model)
{
    var dbDepartment = (from Department d in db.Department
                        where d.Id == id
                        select d).FirstOrDefault() as Department

    dbDepartment.Name = Model.Name;
    dbDepartment.Color = Model.Color;
    // etc, assigning values manually...

    try
    {
        db.SaveChanges();
    }
    catch (Exception ex)
    {
        // oops...
    }
    return RedirectToAction("Index");
}