使用ViewModel在資料庫中保存MVC下拉串列項

我對 MVC 架構完全陌生，需要幫助將下拉串列中的所選專案保存在資料庫中。基本上對于其創建視圖上的每個專案模型，我需要有一個下拉串列，我需要在其中選擇一個客戶端（客戶端名稱顯示在 Dropdonwlist 中）。根據選擇的客戶端，ClientID 應該通過 Post 方法（作為外鍵）鏈接到資料庫中的 Project 物件實體。

GET 部分來顯示Create和填充Dropdownlist作品，POST 是折磨我的東西。

這是我的專案模型：

    public class Project
{
    [Key]
    public int ProjectId { get; set; }
    public string Name { get; set; }
    public string Budget { get; set; }
    public string BusinessCase { get; set; }
    [DataType(DataType.Date)]
    public string StartDate { get; set; }
    [DataType(DataType.Date)]
    public string FinishDate { get; set; }
    public int ClientId { get; set; }
    public Client Client { get; set; }
    public ICollection<ProjectMember> ProjectMembers { get; set; }

    public Project()
    {
    }
}

這是我的客戶端模型：

    public class Client
{
    [Key]
    public int ClientId { get; set; }
    public string Name { get; set; }
    public string State { get; set; }
    public string Address { get; set; }
    public int VAT { get; set; }
    public ICollection<Project> Projects { get; set; }

    public Client()
    {

    }
}

這是我的 CreateProject ViewModel（可能不需要某些屬性 - 我已經嘗試了很多方法......）

public class CreateProjectViewModel
{
    [Key]
    public int ProjectId { get; set; }
    public string Name { get; set; }
    public string Budget { get; set; }
    public string BusinessCase { get; set; }
    [DataType(DataType.Date)]
    public string StartDate { get; set; }
    [DataType(DataType.Date)]
    public string FinishDate { get; set; }
    public int ClientId { get; set; }
    public Client Client { get; set; }
    public ICollection<ProjectMember> ProjectMembers { get; set; }


    public IEnumerable<SelectListItem> Clients { get; set; }
}

這是我用于創建的控制器 GET 方法（使用從客戶端表中選擇值填充下拉串列）：

   public IActionResult Create()
    {


        var clients = _context.Client.Select(r => r.Name);
        var viewmodel = new CreateProjectViewModel
        {
            Clients = new SelectList(clients)
        };
        
        return View(viewmodel); 

    }

And Finally, this is is the Create View using the CreateProjectViewModel:

@model ProjectPortfolioApp.ViewModels.CreateProjectViewModel
    
    @{
        ViewData["Title"] = "Create";
    }



<div class="row">
    <div class="col-md-4">
        <form asp-action="Create">
            <div asp-validation-summary="ModelOnly" class="text-danger"></div>

            <div class="form-group">

                @Html.LabelFor(model => model.Client.Name, htmlAttributes: new { @class = "control label col-md-2" })
                **@Html.DropDownListFor(model => model.ClientId, Model.Clients, "---Select Client---", new { @class = "form-control" })**
            </div>

The above code works fine for displaying the Dropdown elements as expected (Name of the client) using public IEnumerable<SelectListItem> Clients { get; set; }. What I am struggling with is the POST part where I receive error Object reference not set to an instance of an object. When I post model => model.ClientId If I pass here basically anyhing else e.g model => model.Name there is no error only the Client ID is not succesfully posted in database (obviously).

Here is the snippet of HTTP Post which is most probably wrong:

[ValidateAntiForgeryToken]
    public async Task<ActionResult> Create(CreateProjectViewModel model)
    {
        if (ModelState.IsValid)
        {
            var project = new Project() { Name = model.Name, ClientId = model.ClientId, Budget = model.Budget, BusinessCase = model.BusinessCase, StartDate = model.StartDate, FinishDate = model.FinishDate };
            _context.Add(project);
            await _context.SaveChangesAsync();
            return RedirectToAction(nameof(Index));
        }
       
        return View();

        
    }

Can anyone look at my code and help me identify:

• Is my GET Create constructed fine to serve the purpose of POST Create
• What should be passed in @Html.DropDownListFor as valid parameters to avoid the error?
• What should the HTTP Post Create look like?

I looked in dozens of threads, unfortunately nothing helped. Really any help would be appreciated. Thanks.

uj5u.com熱心網友回復：

你必須修復動作

public IActionResult Create()
{

 var clients = _context.Client
           .Select(r =>  new SelectListItem{ Value= r.ClientId.ToString(), Text= r.Name)
           .ToList();

        var viewmodel = new CreateProjectViewModel
        {
            Clients = clients
             .... another properties
        };
        
        return View(viewmodel); 

    }

并查看

@Html.DropDownListFor(model => model.ClientId, @Model.Clients, "---Select Client---", new { @class = "form-control" })

標籤：c# asp.net-mvc model-view-controller viewmodel html.dropdownlistfor

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