EF在编辑时不保存虚拟属性

Question

I have a model Team which looks like this:

public class Team
{
    [Key]
    public int TeamId { get; set; }
    public string Name { get; set; }
    public string Code { get; set; }
    public virtual Group Group { get; set; }
}

I have a method to Edit it:

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Edit(Team team)
{
    using (var db = new DbConnection())
    {
        if (ModelState.IsValid)
        {
            db.Entry(team).State = EntityState.Modified;

            db.Groups.Attach(team.Group);

            db.SaveChanges();
        }

        return PartialView(team);
    }
}

However it is not saving the change to the Group column. Meanwhile Create method does work although I see no difference between them:

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Create(Team team)
{
    using (var db = new DbConnection())
    {                
        if (ModelState.IsValid)
        {
            db.Groups.Attach(team.Group);

            db.Teams.Add(team);
            db.SaveChanges();
            return RedirectToAction("Index");
        }

        return PartialView(team);
    }
}

The values for the Group come the same in both methods. I bring the values to the form like this (again, same thing for both):

ViewBag.Groups = db.Groups.ToList().Select(g => new SelectListItem()
{
    Text = g.Code,
    Value = g.GroupId.ToString(),
    Selected = false
}).ToList(); 

and use them like this:

@Html.DropDownListFor(model => model.Group.GroupId, (List<SelectListItem>)ViewBag.Groups)

Can someone please explain what the difference between the two methods is (in terms of one saving Group and other - not)?

PS Using both the latest MVC and EF.

Answer 1

Apparently it is a must (?) (at least I couldn't find anything better) to add a field matching the type of the Group in order to be able to easily save. No need to attach anything this way. Now my model looks like:

public class Team
{
    [Key]
    public int TeamId { get; set; }
    public string Name { get; set; }
    public string Code { get; set; }
    public int? GroupId { get; set; }
    public virtual Group Group { get; set; }
}

and my Save and Create methods got this line:

db.Groups.Attach(team.Group);

removed.