C# DropDownListFor将值为NULL的post值发送到数据库

我在SQL中有一个课程表和一个洞表。孔表中有一个外键设置为CourseId

这样，当我创建一个孔时，我可以将其指定给相关课程

在我看来，我有一个DropDownList for，它有一个创建孔时可供选择的球场列表。但是，当我尝试发回值时，courseID不会发回

HoleViewModel

 // GET: HoleViewModels/Create
    public ActionResult Create()
    {
        var dbcourse = db.Course.ToList();

        //Make selectlist, which is IEnumerable<SelectListItem>
        var courseNameDropdownList = new SelectList(db.Course.Select(item => new SelectListItem()
        {
            Text = item.CourseName.ToString(),
            Value = item.CourseId.ToString()
        }).ToList(), "Value", "Text");

        // Assign the Selectlist to the View Model   
        var viewCourse = new HoleViewModel()
        {
            Course = dbcourse.FirstOrDefault(),
            // The Dropdownlist values
            CourseNamesDropdownList = courseNameDropdownList,

        };

        
        return View(viewCourse);
    }

    // POST: HoleViewModels/Create
    // To protect from overposting attacks, please enable the specific properties you want to bind to, for 
    // more details see https://go.microsoft.com/fwlink/?LinkId=317598.
    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Create([Bind(Include = "HoleId,HoleNumber,Par,Length,StrokeIndex,CourseId")]  HoleViewModel holeViewModel)
    {
        if (ModelState.IsValid)
        {
            db.HoleViewModels.Add(holeViewModel);
            db.SaveChanges();
            return RedirectToAction("Index");
        }

        
        return View(holeViewModel);
    }

{ @Html.AntiForgeryToken（）

HoleViewModel

@Html.ValidationSummary（true，“，new{@class=“text danger”}）
@LabelFor（model=>model.HoleNumber，htmlAttributes:new{@class=“controllabel col-md-2”}）
@EditorFor（model=>model.HoleNumber，new{htmlAttributes=new{@class=“form control”}}）
@Html.ValidationMessageFor（model=>model.HoleNumber，“，new{@class=“text danger”}）
@ HTML（LabelFor）（Model＝> model .PAR，HTMLATP：新的{@类=“控制标签COL-MD-2”}）
@ html（EditorFor）（model = > model .PAR，新{HTMLATRATITES =新{{Cype＝“窗体控件”}）
@ html（ValidationMessageFor）（model = > model .PAR，“”，新{@类=“文本危险”}）
@LabelFor（model=>model.Length，htmlAttributes:new{@class=“controllabel col-md-2”}）
@EditorFor（model=>model.Length，new{htmlAttributes=new{@class=“form control”}）
@Html.ValidationMessageFor（model=>model.Length，“，new{@class=“text danger”}）
@LabelFor（model=>model.StrokeIndex，htmlAttributes:new{@class=“controllabel col-md-2”}）
@EditorFor（model=>model.StrokeIndex，new{htmlAttributes=new{@class=“form control”}}）
@Html.ValidationMessageFor（model=>model.StrokeIndex，“，new{@class=“text danger”}）
@LabelFor（model=>model.Course.CourseId，“CourseId”，htmlAttributes:new{@class=“controllabel col-md-2”}）
@Html.DropDownListFor（model=>model.Course.CourseId，model.CourseNamesDropdownList，“请从列表中选择”，新建{@class=“form control”}）
@Html.ValidationMessageFor（model=>model.CourseId，“，new{@class=“text danger”}）

这样，当HaveVIEW发布回HONENUM、PAR、长度、SPLKEDISTIC、CURIDID时，它将所有数据传递到孔表之外，而不是它所发布的NULL ID。 要将CourseId输入数据库，我在帖子中遗漏了什么

@using (Html.BeginForm()) 

<div class="form-horizontal">
    <h4>HoleViewModel</h4>
    <hr />
    @Html.ValidationSummary(true, "", new { @class = "text-danger" })
    <div class="form-group">
        @Html.LabelFor(model => model.HoleNumber, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.HoleNumber, new { htmlAttributes = new { @class = "form-control" } })
            @Html.ValidationMessageFor(model => model.HoleNumber, "", new { @class = "text-danger" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.Par, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.Par, new { htmlAttributes = new { @class = "form-control" } })
            @Html.ValidationMessageFor(model => model.Par, "", new { @class = "text-danger" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.Length, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.Length, new { htmlAttributes = new { @class = "form-control" } })
            @Html.ValidationMessageFor(model => model.Length, "", new { @class = "text-danger" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.StrokeIndex, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.StrokeIndex, new { htmlAttributes = new { @class = "form-control" } })
            @Html.ValidationMessageFor(model => model.StrokeIndex, "", new { @class = "text-danger" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.Course.CourseId, "CourseId", htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.DropDownListFor(model => model.Course.CourseId, Model.CourseNamesDropdownList, "Please select from List", new { @class = "form-control" })
            @Html.ValidationMessageFor(model => model.CourseId, "", new { @class = "text-danger" })
        </div>
    </div>