添加新实体不会链接到现有的相关实体，而是创建一个新实体

Question

My problem is the following: I have a "Create new Game" form. Within that form, there is a dropdown populated with data (categories). So each game belongs to one category and both games and categories are stored in a database. However, the Insert of a game doesnt link that game to the selected category. Instead, a completely new, nameless category is created and assigned to that game.

In the .cshtml file:

<div class="editor-field">
@Html.DropDownListFor(model => model.Category.Id, GameManager.Controllers.GameController.AllCategories())
@Html.ValidationMessageFor(model => model.Category)
</div>

the game model:

public class Game
{ 

    public int Id { get; set; }
    public String Name { get; set; }
    public virtual Studio Studio { get; set; }
    public virtual Category Category { get; set; }
    public float Price { get; set; }
    public int Downloads { get; set; }
    public DateTime ReleaseDate { get; set; }

}

the category model:

public class Category
{
    public int Id { get; set; }
    public String Name {get; set;}
}

Edit: the controller

[HttpPost]
public ActionResult Create(Game game)
{
    if (ModelState.IsValid)
    {
        db.Games.Add(game);
        db.SaveChanges();
        return RedirectToAction("Index");
    }

    return View(game);
}

Answer 1

You need to load the category:

game.Category = db.Categories.Single(c => c.Id == game.Category.Id);
db.Games.Add(game);