我的视图没有返回下拉列表信息

Question

I am running the Visual studio 2012 debugger, and have found that my productViewModelList parameter does not contain all the values I want my view to pass to my [HTTPPOST] Edit action. I don't understand why. Please refer to the comment in the code sample below for the location I that I inserted the breakpoint and checked the values of productViewModelList.

The following values are given to productViewModelList:

BrandId = 0, BrandName = "6", BrandSelectListItem = null, ID = 5, Name = "Crutch", Price = 10.0

• BrandID is incorrect, In the view in my DropDownList, I assign "Catatonics Inc." Which has an ID of 6, Which I verified in my database.
• BrandName is showing "6" which should be in BrandID, BrandName should be "Catatonics Inc."
• BrandSelectList item is an object of type SelectListItem, it contains the values that go into The DropDownList item in my view. The DropDownList correctly shows the values, but BrandSelectList is null when my [httpPost] edit action executes. I need to access the DropDownList's Selected item.
• all other values, ID, Name, and Price are correct.

Here are some classes in my code.

MedicalProductController

public class MedicalProductController : Controller
{
    private MvcMedicalStoreDb _db = new MvcMedicalStoreDb();

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Edit(IEnumerable<MedicalProductViewModel> productViewModelList)
    {
        // I have a breakpoint inserted here, and check productViewModelList with debugger.  

        var modelList = GetMedicalProductList(productViewModelList);
        if (ModelState.IsValid)
        {
            foreach (var model in modelList)
                _db.Entry(model).State = EntityState.Modified;

            _db.SaveChanges();
            return RedirectToAction("Index");
        }

        return View(productViewModelList);
    }
}

MedicalProductMapper

public class MedicalProductMapper
{

    public IEnumerable<MedicalProductViewModel> MapMedicalProductViewModel(IEnumerable<MedicalProduct> productList, IEnumerable<Brand> brandList)
    {
        var brandSelectListItem = brandList.Select(b => new SelectListItem()
                                                {
                                                    Text = b.Name,
                                                    Value = b.ID.ToString()
                                                });

        var viewModelList = productList.Select(p => new MedicalProductViewModel() 
                                {
                                    BrandID = p.BrandID,
                                    BrandName = brandList.SingleOrDefault(b => b.ID == p.BrandID).Name,
                                    BrandSelectListItem = brandSelectListItem,
                                    ID = p.ID,
                                    Price = p.Price,
                                    Name = p.Name
                                });

        return viewModelList;
    }

    public IEnumerable<MedicalProduct> MapMedicalProductList(IEnumerable<MedicalProductViewModel> viewModelList)
    {
        var modelList = viewModelList.ToArray().Select( viewModel => new MedicalProduct()
        {
            Name = viewModel.Name,
            Price = viewModel.Price,
            BrandID = Convert.ToInt32(viewModel.BrandSelectListItem.Select(b => b.Value.ToString()))
        });

        return modelList;
    }
}

EDIT.cshtml

@model IEnumerable<MvcMedicalStore.Models.MedicalProductViewModel>

@{
    ViewBag.Title = "Edit";
}

<h2>Edit</h2>

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

    <fieldset>
        <legend>MedicalProduct</legend>
        @Html.EditorFor(m => m)        
        <p>
            <input type="submit" value="Save" />
        </p>
    </fieldset>
}

<div>
    @Html.ActionLink("Back to List", "Index")
</div>

@section Scripts {
    @Scripts.Render("~/bundles/jqueryval")
}

EditorTemplates/MedicalProductViewModel.cshtml

(this is in subdirectory of Edit.cshtml's directory.)

@model MvcMedicalStore.Models.MedicalProductViewModel

@Html.HiddenFor(item => Model.ID)

<div class="editor-label">
    @Html.LabelFor(item => Model.Name)
</div>
<div class="editor-field">
    @Html.EditorFor(item => Model.Name)
    @Html.ValidationMessageFor(item => Model.Name)
</div>

<div class="editor-label">
    @Html.LabelFor(item => Model.Price)
</div>
<div class="editor-field">
    @Html.EditorFor(item => Model.Price)
    @Html.ValidationMessageFor(item => Model.Price)
</div>

<div class="editor-label">
    @Html.LabelFor(item => Model.BrandName)
</div>
<div class="editor-field">
    @Html.DropDownListFor(item => Model.BrandName, Model.BrandSelectListItem)
    @Html.ValidationMessageFor(item => Model.BrandName)
</div>

EDIT:

Brand

public class Brand
{
    [Key]
    public int ID { get; set; }

    [Required]
    [StringLength(30)]
    public string Name { get; set; }
}

MedicalProduct

public class MedicalProduct
{
    [Key]
    public int ID { get; set; }

    [Required]
    [StringLength(50)]
    public string Name { get; set; }

    [Required]
    [DataType(DataType.Currency)]
    public double Price { get; set; }

    // is a foreign key
    public int BrandID { get; set; }
}

MedicalProductViewModel

public class MedicalProductViewModel
{
    [Key]
    public int ID { get; set; }

    [Required]
    [StringLength(50)]
    public string Name { get; set; }

    [Required]
    [DataType(DataType.Currency)]
    public double Price { get; set; }

    public int BrandID { get; set; }

    [DisplayFormat(NullDisplayText="[Generic]")]
    public string BrandName { get; set; }

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

Answer 1

You don't have a form element for BrandID , so when the model binder translates the form POST into a model it's going to use the default value for int , which is 0. You do have a form element for BrandName :

@Html.DropDownListFor(item => Model.BrandName, Model.BrandSelectListItem)

This is presumably (depending on the definition of BrandSelectListItem , and based on the observed behavior you describe) going to create a select element where the option elements use the available BrandName values as the text and the available BrandID values as the value. When a select element is included in an HTTP POST, the selected value is what is posted. So you're getting the selected BrandID , but assigning it to the value for BrandName by binding it to that property on the model.

It sounds like what you really want is this:

@Html.DropDownListFor(item => Model.BrandID, Model.BrandSelectListItem)

This would then give you:

BrandId = 6, BrandName = "", BrandSelectListItem = null, ID = 5, Name = "Crutch", Price = 10.0

You still don't have a BrandName value, but do you really need one in this case? It's duplicated data. The model can dynamically determine the BrandName from the known BrandID . So BrandName doesn't need to be a read/write property on the model, it can just be a read-only property which fetches the BrandName based on the model's BrandID . It might be something as simple as:

public string BrandName
{
    get
    {
        var brandName = GetBrandName(BrandID);
        return brandName;
    }
}

In this case you'd implement GetBrandName to fetch the value from the data store. Or do the whole thing in-line if it's clear and concise enough, that's up to you. You could also cache the value in a class-level private variable so that it only needs to be fetched once. (Just make sure to clear out that variable or re-fetch it any time BrandID changes.)

You could keep both properties as they are, but then you'd be responsible for making sure they stay synchronized for the life of the object, which includes the time it spends in the UI. Any time you have the same piece of information recorded in two places, you have the responsibility of keeping it synchronized. Which is never fun. In this case you'd need two elements in the view (the second one could be a hidden form field, or perhaps a drop down list styled to be hidden), and you'd have to write some JavaScript to update one any time the other one changes.