Entity Framework with ASP.NET MVC

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");
}

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.

How about this

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Edit(int id, Department dept)
{
    Department tmp = new Department()
    {
        Id = dept.Id
    };
    try
    {
        db.Departments.Attach(dept, tmp);
        db.Save();
    }
}

Maybe the tmp-part can be exchanged for a call to UpdateModel after the Attach part, but I'm not sure.

Edit: Just looked up the documentation on Attach(Entity e, Entity original) and it does indeed replace original with e (or update original accordingly or w/e).

I don not recommend to use a POCO into you MVC application. You should use a DTO or a model.

Maybe you should retrieve an object by id before and then replace the values you supplied by arguments to edit method and then save changes. You get an error because you didn't deattach the model in the first place.

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Edit(Guid id, Department Model) {
var entity=from m in db.Models where m.Id == Model.id select m;
Model entityAttached=entity.FirstOrDefault();
//replace all the new values in you object
db.SaveChanges();
}

or deattach object when you get it by id and attach it back when you want to save the new values in edit method.