我有以下问题，我创建了一个应用程序添加游戏类别和游戏本身到数据库。我创建了一个关系，不幸的是，当我添加到数据库时，我得到了一个错误。
一个月一个月一个月一个月一个月
游戏类别型号：

using System.Collections.Generic;

namespace relationship.Models
{
    public class GameCategory
    {
        public int Id { get; set; }
        public string Name { get; set; }

        public ICollection<Game> Game { get; set; }
    }
}

游戏模型：

namespace relationship.Models
{
    public class Game
    {
        public int GameId { get; set; }
        public string Name { get; set; }

        public GameCategory Category { get; set; }
        public int CategoryId { get; set; }
    }
}

视图模型：

using relationship.Models;
using System.ComponentModel.DataAnnotations;
using System.Collections.Generic;
using Microsoft.AspNetCore.Mvc.Rendering;
namespace relationship.ViewModels
{
    public class AddGameViewModel
    {     
        [Required]
        public string GameName { get; set; }
        public int CategoryID { get; set; }

        public List<SelectListItem> Categories { get; set; }

        public AddGameViewModel(IEnumerable<GameCategory> categories)
        {
            Categories = new List<SelectListItem>();
            foreach (var catData in categories)
            {
                Categories.Add(new SelectListItem { Text = catData.Name.ToString(), Value = catData.Id.ToString() });
            }
            return;
        }
    }
}

游戏存储库：

using System.Collections.Generic;
using System.Linq;

namespace relationship.Models
{
    public class GameRepository : IGameRepository
    {
        private readonly AppDbContext appDbContext;
        public GameRepository(AppDbContext dbContext)
        {
            appDbContext = dbContext;
        }

        public void AddGame(Game game)
        {
            appDbContext.Games.Add(game);
            appDbContext.SaveChanges();
        }

        public IEnumerable<Game> Games()
        {
            return appDbContext.Games.ToList();
        }
    }
}

最后是游戏控制器：

using Microsoft.AspNetCore.Mvc;
using relationship.Models;
using relationship.ViewModels;

namespace relationship.Controllers
{
    public class GameController : Controller
    {
        private readonly IGameRepository gameRepository;
        private readonly ICategoryRepository categoryRepository;

        public GameController(IGameRepository gameRepo, ICategoryRepository catRepo)
        {
            gameRepository = gameRepo;
            categoryRepository = catRepo;
        }

        public IActionResult Index()
        {
            return View();
        }

        [HttpGet]
        public IActionResult Add()
        {
            var addGameViewModel = new AddGameViewModel(categoryRepository.GameCategory());
            return View(addGameViewModel);
        }

        [HttpPost]
        public IActionResult Add(AddGameViewModel addGameViewModel)
        {
            if (ModelState.IsValid)
            {
                GameCategory gameCategory = categoryRepository.GetDetails(addGameViewModel.CategoryID);

                if(gameCategory == null)
                {
                    return NotFound();
                }

                Game game = new Game
                {
                    Name = addGameViewModel.GameName,
                    Category = gameCategory
                };

                gameRepository.AddGame(game);
                return RedirectToAction("Index");
            }
            return View(addGameViewModel);
        }
    }
}

我不知道出了什么问题。
我的错误屏幕：