MVC4: Modelstate is not valid when creating a new record

Question

I try to add a new Country which has a link to continent. When I press the "Create" button, it doesn't add a new record. I debugged my project and I think it's because the ValidState is false. The reason because of this is that the property "Continent" is null, but the Continent_Id isn't. I have the same problem when I try to edit an existing Country. (I have populated my database with an SQL script in SQL Management Studio)

Can someone help me please?

Continent class:

public class Continent
{
    public int Id { get; set; }
    [Required, MaxLength(25)]
    public string Name { get; set; }

    //Navigation
    public virtual List<Country> Countries { get; set; }
}

Country class

public class Country
{
    public int Id { get; set; }
    [Required, MaxLength(25)]
    public string Name { get; set; }
    [MaxLength(5)]
    public string Abbreviation { get; set; }

    public int Continent_Id { get; set; }

    //Navigation
    [Required, ForeignKey("Continent_Id")]
    public virtual Continent Continent { get; set; }

}

Controller class ( create function )

  //
    // GET: /Countries/Create

    public ActionResult Create()
    {
        ViewBag.Continent_Id = new SelectList(db.Continents, "Id", "Name");
        return View();
    }

    //
    // POST: /Countries/Create

    [HttpPost]
    public ActionResult Create(Country country)
    {
       var errors = ModelState.Values.SelectMany(v => v.Errors); //to check the errors
        if (ModelState.IsValid)
        {
            db.Countries.Add(country);
            db.SaveChanges();
            return RedirectToAction("Index");
        }


        ViewBag.Continent_Id = new SelectList(db.Continents, "Id", "Name", country.Continent_Id);
        return View(country);

Answer 1

I fixed this issue by putting the Required validation off of Continent, and set it only at the Continent_Id. Now the ID property is required, but the Continent isn't.

public class Country
{
public int Id { get; set; }
[Required, MaxLength(25)]
public string Name { get; set; }
[MaxLength(5)]
public string Abbreviation { get; set; }

[Required] //added Required
public int Continent_Id { get; set; }

//Navigation
[ForeignKey("Continent_Id")] //removed Required
public virtual Continent Continent { get; set; }

}

Thanks for the responses !