How do I make a connected Dropdownlist with a model property?

Question

I have my class:

public class Topico
{
    public int Id { get; set; }
    public string Nome { get; set; }
    public string Observacao { get; set; }
}

and my Subtopic class

public class SubTopico
{
    public int Id { get; set; }
    public virtual Topico Topico { get; set; }
    public int? TopicoId { get; set; }
    public string Nome { get; set; }
    public string Observacao { get; set; }
}

On my controller I have:

public ActionResult Create()
{
    ViewBag.Topico = new SelectList(db.Topicoes, "Id", "Nome");
    return View();
}

and in my view I have my DropDownList

 @Html.DropDownList("Topico",string.Empty)

Until then it carries all topics, etc.

But I can’t make a bind in it by saving it. In my action to save

public ActionResult Create([Bind(Include="Id,Topico,Nome,Observacao")] SubTopico subtopico)
{
    if (ModelState.IsValid)
    {
        subtopico.TopicoId = Convert.ToInt32(Request.Params["Topico"]);
        db.SubTopicoes.Add(subtopico);
        db.SaveChanges();
        return RedirectToAction("Index");
    }

    return View(subtopico);
}

It does not come Topic Id, even because the class is null. So I capture using Request.Params["Topico"];

How do I link my dropdownlist to my property public virtual Topico that I have in my Subtopic class

Answer 1

The way you did, the first time of questioning, it would be like this:

public ActionResult Create()
{
    ViewBag.Topico = new SelectList(db.Topicoes, "Id", "Nome");
    return View();
}

Na View:

@Html.DropDownList("TopicoId", ViewBag.Topico as SelectList)

or

@Html.DropDownListFor(model => model.TopicoId, ViewBag.Topico as SelectList)

Answer 2

Use @DropDownListFor:

@Html.DropDownList(model => model.TopicoId, ((IEnumerable)ViewBag.Topicos).Select(option => new SelectListItem {
    Text = option.Nome, 
    Value = option.Id.Value, 
    Selected = (Model != null) && (option.Id == (int)Model.TopicoId ?? 0)
}, "Selecione..."))

So you don’t need to use Request.Params, that it’s not exactly safe.

In this case the POST, Topico will come null even. It is loaded in the selection of data, as for example if you detail the Subtopico using

context.Subtopicos.Include(st => st.Topico).FirstOrDefault();

Finally, you can simplify your Controller for:

public ActionResult Create([Bind(Include="Id,TopicoId,Nome,Observacao")] SubTopico subtopico)
{
    if (ModelState.IsValid)
    {
        db.SubTopicoes.Add(subtopico);
        db.SaveChanges();
        return RedirectToAction("Index");
    }

    return View(subtopico);
}

Answer 3

One simple way I found and got it was:

In my controller, return a list:

ViewBag.Topicos = db.Topicoes.ToList();

And in my View use Dropdownlistfor as follows.

@Html.DropDownListFor(model => model.TopicoId, new SelectList(ViewBag.Topicos,"Id","Nome","Selecione..."))

Answer 4

It is also possible to use SQL query as data source for Dropdownlist:

No Controller:

public ActionResult NomeDaAction() {
    ViewBag.DadosDropDown = (from c in db.Tabela select c.ColunaDaTabela).Distinct();
    Return View();
}

In the View:

@Html.DropDownListFor(model => model.CampoOndeArmazenar, new SelectList(ViewBag.DadosDropDown, "Coluna1DaTabela", "OutraColunaDaTabela"), htmlAttributes: new { @class = "ClasseCSSdoDropDownList" })

Since, in the view, if you don’t want to assign a CSS class to the checkbox, simply remove the htmlAttributes command as below:

@Html.DropDownListFor(model => model.CampoOndeArmazenar, new SelectList(ViewBag.DadosDropDown, "Coluna1DaTabela", "OutraColunaDaTabela"))