Estoy tratando de obtener una lista desplegable que funcione para los usuarios que están siendo calificados. Cada usuario puede tener múltiples graduaciones. Entonces, cuando creo una nueva calificación, quiero un menú desplegable para especificar qué uso quién recibirá la calificación.

Sigo recibiendo:

No hay ningún elemento ViewData de tipo 'IEnumerable' que tenga la clave 'GradingId'.

He examinado muchas otras preguntas, pero no puedo resolver lo que necesito cambiar en mi controlador, vista o modelos.

GradingController.cs

 public ActionResult Create()
        {
            return View();
        }

        // POST: Gradings/Create
        [HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult Create([Bind(Include = "GradingId,Eye,Cheek,Mouth,RestSymmetryTotal,RestSymmetryScore,VolForeheadWrinkle,VolGentleEyeClosure,VolOpenMouthSmile,VolSnarl,VolLipPucker,VolSymmetryTotal,VolSymmetryScore,SynForeheadWrinkle,SynGentleEyeClosure,SynOpenMouthSmile,SynSnarl,SynLipPucker,SynkinesisScore,CompositeScore")] Grading grading)
        {
            if (ModelState.IsValid)
            {
                grading.GradeDate = DateTime.Now;
                db.Gradings.Add(grading);
                db.SaveChanges();
                return RedirectToAction("Index");
            }
            ViewBag.GradingId = new SelectList(db.Gradings, "GradingId", "CodeName");
            return View(grading);
        }

        // GET: Gradings/Edit/5
        public ActionResult Edit(int? id)
        {
            if (id == null)
            {
                return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
            }
            Grading grading = db.Gradings.Find(id);
            if (grading == null)
            {
                return HttpNotFound();
            }
            ViewBag.GradingId = new SelectList(db.Gradings, "GradingId", "CodeName");
            return View(grading);
        }

        // POST: Gradings/Edit/5
        [HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult Edit([Bind(Include = "GradingId,Eye,Cheek,Mouth,RestSymmetryTotal,RestSymmetryScore,VolForeheadWrinkle,VolGentleEyeClosure,VolOpenMouthSmile,VolSnarl,VolLipPucker,VolSymmetryTotal,VolSymmetryScore,SynForeheadWrinkle,SynGentleEyeClosure,SynOpenMouthSmile,SynSnarl,SynLipPucker,SynkinesisScore,CompositeScore")] Grading grading)
        {
            if (ModelState.IsValid)
            {
                db.Entry(grading).State = EntityState.Modified;
                db.SaveChanges();
                return RedirectToAction("Index");
            }
            ViewBag.GradingId = new SelectList(db.Gradings, "GradingId", "CodeName");
            return View(grading);
        }

Create.cshtml (Ver)

@model FaceToFace.Model.Grading

    <h2>Create</h2>


    @using (Html.BeginForm())
    {
        @Html.AntiForgeryToken()

        <div class="editor-label">
            @Html.LabelFor(model => model.User.CodeName, "User Name")
        </div>
        <div class="editor-field">
            @Html.DropDownList("GradingId", String.Empty)
        </div>
            <div class="form-group">
                <div class="col-md-offset-2 col-md-10">
                    <input type="submit" value="Create" class="btn btn-default" />
                </div>
            </div>
        </div>
    }

grading.cs (modelo)

namespace FaceToFace.Model
{
    using System;
    using System.Collections.Generic;
    using System.ComponentModel.DataAnnotations;
    using System.ComponentModel.DataAnnotations.Schema;
    using System.Data.Entity.Spatial;

    [Table("Grading")]
    public partial class Grading
    {
public int? User_UserID { get; set; }
        public virtual User User { get; set; }

        [DatabaseGenerated(DatabaseGeneratedOption.None)]
        public int GradingId { get; set; }

        public DateTime GradeDate { get; set; }
        public DateTime GradeEditDate { get;, set; }
    }
}

User.cs (modelo)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace FaceToFace.Model
{
    public class User
    {
      public virtual ICollection<Grading> UserGradings { get; set; }   
    }
}