How should I add a database record with user field set to currently logged user ASP.NET MVC

Question

I have a problem while adding a category to the logged user. I have an error System.InvalidOperationException: "Sequence has no elements." .

My code:

public class AppUser
{
    public int Id { get; set; }
    public string Nickname { get; set; }
    public string Name { get; set; }
    public string Surname { get; set; }
    public DateTime BirthDate { get; set; }
    public virtual ICollection<Category> Categories { get; set; }

}

public class Category
{
    public int Id { get; set; }
    public string Name { get; set; }
    public AppUser AppUser { get; set; }
    public int AppUserId { get; set; }

}

public class CategoryAndAppUserViewModel
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int AppUserId { get; set; }
}

// GET: Categories/Create
public ActionResult Create()
{
    return View();
}

My controller:

// GET: Categories/Create
public ActionResult Create()
{
    return View();
}

// POST: Categories/Create
[Authorize]
[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Create(CategoryAndAppUserViewModel category)
{
    if (ModelState.IsValid)
    {

        string userId = User.Identity.GetUserId();
        AppUser appUser = _context.AppUsers.Single(u => u.Name == userId);
        var categoryVM = new Category { Name = category.Name, AppUser = appUser };
        appUser.Categories.Add(categoryVM);
        _context.SaveChanges();
        return RedirectToAction("Index");
    }

    return View(category);
}

I want to do: When the user is logged in, goes to My Examples , create a new Category , fill the new category name, the app must automatically add the new category to the logged in user. The main problem is to get the logged user id.

Answer 1

I have a problem while adding a category to the logged user. I have an error System.InvalidOperationException: "Sequence has no elements." .

My code:

public class AppUser
{
    public int Id { get; set; }
    public string Nickname { get; set; }
    public string Name { get; set; }
    public string Surname { get; set; }
    public DateTime BirthDate { get; set; }
    public virtual ICollection<Category> Categories { get; set; }

}

public class Category
{
    public int Id { get; set; }
    public string Name { get; set; }
    public AppUser AppUser { get; set; }
    public int AppUserId { get; set; }

}

public class CategoryAndAppUserViewModel
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int AppUserId { get; set; }
}

// GET: Categories/Create
public ActionResult Create()
{
    return View();
}

My controller:

// GET: Categories/Create
public ActionResult Create()
{
    return View();
}

// POST: Categories/Create
[Authorize]
[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Create(CategoryAndAppUserViewModel category)
{
    if (ModelState.IsValid)
    {

        string userId = User.Identity.GetUserId();
        AppUser appUser = _context.AppUsers.Single(u => u.Name == userId);
        var categoryVM = new Category { Name = category.Name, AppUser = appUser };
        appUser.Categories.Add(categoryVM);
        _context.SaveChanges();
        return RedirectToAction("Index");
    }

    return View(category);
}

I want to do: When the user is logged in, goes to My Examples , create a new Category , fill the new category name, the app must automatically add the new category to the logged in user. The main problem is to get the logged user id.