如何从一个表中获取 Id 并将其保存到 ASP.NET Core 中的另一个表，存储库模式

Question

This is Schedule Exam service:

public int AddSchedule(ScheduleExamViewModel schedule)
{
        var newSchedule = new ScheduleExam()
        {
            ExamDate = schedule.ExamDate,
            SubjectId = schedule.SubjectId,
            StudentId = schedule.StudentId,
            Status = schedule.Status
        };

        _context.ScheduleExams.Add(newSchedule);
        _context.SaveChanges();

        return newSchedule.Id;
}

This is Schedule exam controller :

// GET: ScheduleExamController/Create
public IActionResult Create()
{
    var model = new ScheduleExamViewModel();

    ViewBag.Subject = new SelectList(_isubject.GetSubject(), "Id", "Name");
    ViewBag.Student = new SelectList(_istudent.GetStudent(), "Id", "Name");

    return View(model);
}

// POST: ScheduleExamController/Create
[HttpPost]
[ValidateAntiForgeryToken]
public IActionResult Create(ScheduleExamViewModel schedule)
{
    if (ModelState.IsValid)
    {
        int id = _ischeduleExam.AddSchedule(schedule);

        if (id > 0)
        {
            return RedirectToAction(nameof(Create));
        }
    }

    ViewBag.Subject = new SelectList(_isubject.GetSubject(), "Id", "Name");
    ViewBag.Student = new SelectList(_istudent.GetStudent(), "Id", "Name");

    return View(schedule);
}

I want to get the Id of student and save it to table of schedule exam by clicking on the "Schedule New Exam"

This is the index.cshtml of the Student table:

<table class="table table-hover table-bordered table-condensed">
            <thead class="table-color text-white">
                <tr>
                    <th>
                        @Html.DisplayNameFor(model => model.Id)
                    </th>
                    <th>
                        @Html.DisplayNameFor(model => model.Name)
                    </th>
                    <th>
                        @Html.DisplayNameFor(model => model.LastName)
                    </th>
                    <th>
                        @Html.DisplayNameFor(model => model.Email)
                    </th>
                    <th>Schedule New Exam</th>
                    <th>Properties</th>

                </tr>
            </thead>
            <tbody>
                @foreach (var item in Model)
                {
                <tr>
                    <td>
                        @Html.DisplayFor(modelItem => item.Id)
                    </td>
                    <td>
                        @Html.DisplayFor(modelItem => item.Name)
                    </td>
                    <td>
                        @Html.DisplayFor(modelItem => item.LastName)
                    </td>
                    <td>
                        @Html.DisplayFor(modelItem => item.Email)
                    </td>
                    <td>
                        <a asp-action="Create" asp-controller="ScheduleExam", asp-route-id="@item.Id">Schedule New Exam</a>
                    </td>
                    <td>
                        @Html.ActionLink(" ", "Edit", new { id = item.Id }, new { @class = "fa fa-edit", title = "Edit" }) &nbsp;|&nbsp;
                        @Html.ActionLink(" ", "Details", new { id = item.Id }, new { @class = "fa fa-info-circle", title = "More details" }) &nbsp;|&nbsp;
                        @Html.ActionLink(" ", "Delete", new { id = item.Id }, new { @class = "fa fa-trash", title = "Delete" })
                    </td>
                </tr>
                }
            </tbody>
        </table>

and this is the create.csthml page of schedule exam:

 <form asp-action="Create">
        <div asp-validation-summary="ModelOnly" class="text-danger"></div>

        <div class="form-group">
            <label asp-for="ExamDate" class="control-label"></label>
            <input asp-for="ExamDate" class="form-control" />
            <span asp-validation-for="ExamDate" class="text-danger"></span>
        </div>
        <div class="form-group">
        <label asp-for="StudentId" class="control-label"></label>
        <input asp-for="StudentId" class="form-control" value="@ViewBag.model.Id" />
        <span asp-validation-for="StudentId" class="text-danger"></span>
    </div>
        <div class="input-group">
            <div class="input-group-prepend">
                <span class="input-group-text"><strong>Subject:</strong></span>
            </div><br />
            <select asp-for="SubjectId" class="form-control input-hover" asp-items="ViewBag.Subject">
                <option value="">Please choose a subject...</option>
            </select>
            <span asp-validation-for="SubjectId " class="text-danger"></span>
        </div><br />
        <div class="form-group">
            <input type="submit" value="Create" class="btn btn-primary" />
        </div>
    </form>

but when I click on the Schedule New exam in index.cshtml of Student table I get this error:

RuntimeBinderException: Cannot perform runtime binding on a null reference CallSite.Target(Closure , CallSite , object )

CallSite.Target(Closure , CallSite , object ) System.Dynamic.UpdateDelegates.UpdateAndExecute1<T0, TRet>(CallSite site, T0 arg0)
AspNetCore.Views_ScheduleExam_Create.b__22_0() in Create.cshtml +

<input asp-for="StudentId" class="form-control" value="@ViewBag.model.Id" />

Please solve it by details and I'm using ASP.NET Core 3.1 MVC using a repository pattern.

Thank you

Answer 1

first of all, you should get an Id inside Create-Action and set for students :

public IActionResult Create(int Id)
{
    var model = new ScheduleExamViewModel();
    
    ViewBag.Subject = new SelectList(_isubject.GetSubject(), "Id", "Name");
    ViewBag.Student = new SelectList(_istudent.GetStudent(), "Id", "Name" , Id);

    return View(model);
}

Second, I can't find a drop-down list for Students in your view. I think you should set Id in the controller for ViewModel.

var model = new ScheduleExamViewModel();
mode.Id = Id;

Third, you passed a model to the View and you can get information from the Model.

@model ScheduleExamViewModel
<input asp-for="StudentId" class="form-control" value="@Model.Id" />