ASP.NET MVC Core Cascading DropDownList

Question

I'm having trouble finding a tutorial / video that shows how to implement Cascading DropDownList from a Database using EntityFramework. I'm using ASP.NET MVC Core, EntityFramework Core with C#.

As of now, I'm able to retrieve the data from my database to my 3 DropDownList fine.

What I would like to be able to accomplish is to have the user select a State first which would then display all Cities related to that State. Then after user has selected a City it would display the Zip Code(s) related to the City.

Any help would be greatly appreciated.

Models

    public class Customer
{
    public int CustomerId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public int StateId { get; set; }
    public int CityId { get; set; }
    public int ZipId { get; set; }

    public State State { get; set; }
    public City City { get; set; }
    public Zip Zip { get; set; }
}

    public class State
{
    public int StateId { get; set; }
    public string Abbr { get; set; }

    public List<Customer> Customers { get; set; }
}

    public class City
{
    public int CityId { get; set; }
    public string Name { get; set; }

    public int StateId { get; set; }
    public State State { get; set; }

    public List<Customer> Customers { get; set; }
}

    public class Zip
{
    public int ZipId { get; set; }
    public string PostalCode { get; set; }

    public int CityId { get; set; }
    public City City { get; set; }

    public List<Customer> Customers { get; set; }
}

ViewModels

    public class CustomerFormVM
{
    public int CustomerId { get; set; }

    [Display(Name = "First Name")]
    [StringLength(50)]
    public string FirstName { get; set; }

    [Display(Name = "Last Name")]
    [StringLength(50)]
    public string LastName { get; set; }

    [Required(ErrorMessage = "Select State")]
    [Display(Name = "State")]
    public int StateId { get; set; }

    //public IEnumerable<State> States { get; set; }
    public IEnumerable<SelectListItem> States { get; set; }

    [Required(ErrorMessage = "Select City")]
    [Display(Name = "City")]
    public int CityId { get; set; }

    //public IEnumerable<City> Citys { get; set; }
    public IEnumerable<SelectListItem> Citys { get; set; }

    [Required(ErrorMessage = "Select Zip")]
    [Display(Name = "Zip")]
    public int ZipId { get; set; }

    //public IEnumerable<Zip> Zips { get; set; }
    public IEnumerable<SelectListItem> Zips { get; set; }
}

CustomerController

public class CustomerController : Controller
{
    private MultiDbContext db;

    public CustomerController(MultiDbContext context)
    {
        db = context;
    }

    // GET: /<controller>/
    public IActionResult Index()
    {
        return View(db.Customers.ToList());
    }

    public IActionResult getCititesFromDatabaseByStateId(int id)
    {
        return View(db.Citys.Where(c => c.StateId == id).ToList());
    }

    public IActionResult getCities(int id)
    {
        var cities = new List<City>();
        cities = getCititesFromDatabaseByStateId(id); //call repository
        return Json(cities);
    }

    public ActionResult Create()
    {
        var states = db.States.ToList();
        var citys = db.Citys.ToList();
        var zips = db.Zips.ToList();

        var viewModel = new CustomerFormVM
        {
            States = states,
            Citys = citys,
            Zips = zips
        };

        return View(viewModel);
    }


    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Create(CustomerFormVM vm)
    {
        if (ModelState.IsValid)
        {
            var customer = new Customer();
            {
                customer.FirstName = vm.FirstName;
                customer.LastName = vm.LastName;
                customer.StateId = vm.StateId;
                customer.CityId = vm.CityId;
                customer.ZipId = vm.ZipId;
            }
            db.Customers.Add(customer);
            db.SaveChanges();
            return RedirectToAction("Index");
        }

        else
        {
            vm.States = db.States.ToList();
            vm.Citys = db.Citys.ToList();
            vm.Zips = db.Zips.ToList();
            return View(vm);
        }
    }


    public ActionResult Edit(int? id)
    {
        if (id == null)
        {
            return NotFound();
        }

        var customervm = new CustomerFormVM();
        {
            Customer customer = db.Customers.SingleOrDefault(c => c.CustomerId == id);

            if (customer == null)
            {
                return NotFound();
            }

            customervm.CustomerId = customer.CustomerId;
            customervm.FirstName = customer.FirstName;
            customervm.LastName = customer.LastName;

            // Retrieve list of States
            var states = db.States.ToList();
            customervm.States = states;

            // Retrieve list of Citys
            var citys = db.Citys.ToList();
            customervm.Citys = citys;

            // Retrieve list of Citys
            var zips = db.Zips.ToList();
            customervm.Zips = zips;

            // Set the selected state
            customervm.StateId = customer.StateId;

            // Set the selected city
            customervm.CityId = customer.CityId;

            // Set the selected zip
            customervm.ZipId = customer.ZipId;
        }
        return View(customervm);
    }


    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Edit(CustomerFormVM vmEdit)
    {
        if (ModelState.IsValid)
        {
            Customer customer = db.Customers.SingleOrDefault(c => c.CustomerId == vmEdit.CustomerId);

            if (customer == null)
            {
                return NotFound();
            }

            customer.FirstName = vmEdit.FirstName;
            customer.LastName = vmEdit.LastName;
            customer.StateId = vmEdit.StateId;
            customer.CityId = vmEdit.CityId;
            customer.ZipId = vmEdit.ZipId;

            db.Entry(customer).State = EntityState.Modified;
            db.SaveChanges();
            return RedirectToAction("Index");
        }
        return View(vmEdit);
    }
}

Create View

        <div class="form-group">
        @Html.LabelFor(c => c.FirstName)
        @Html.TextBoxFor(c => c.FirstName, new { @class = "form-control" })
    </div>

    <div class="form-group">
        @Html.LabelFor(c => c.LastName)
        @Html.TextBoxFor(c => c.LastName, new { @class = "form-control" })
    </div>

    <div class="form-group">
        @*@Html.LabelFor(s => s.StateId)
            @Html.DropDownListFor(s => s.StateId, new SelectList(Model.States, "StateId", "Abbr"), "", new { @class = "form-control" })
            @Html.ValidationMessageFor(s => s.StateId)*@

        <label asp-for="StateId "></label>
        <select asp-for="StateId " asp-items="Model.States" class="form-control" id="state-target"></select>
        <span asp-validation-for="StateId " class="text-danger"></span>
    </div>

    <div class="form-group">
        @*@Html.LabelFor(ct => ct.CityId)
            @Html.DropDownListFor(ct => ct.CityId, new SelectList(Model.Citys, "CityId", "Name"), "", new { @class = "form-control" })
            @Html.ValidationMessageFor(ct => ct.CityId)*@

        <label asp-for="CityId"></label>
        <select asp-for="CityId" asp-items="Model.Citys" class="form-control" id="city-target"></select>
        <span asp-validation-for="CityId" class="text-danger"></span>
    </div>

    <div class="form-group">
        @Html.LabelFor(z => z.ZipId)
        @Html.DropDownListFor(z => z.ZipId, new SelectList(Model.Zips, "ZipId", "PostalCode"), "", new { @class = "form-control" })
        @Html.ValidationMessageFor(z => z.ZipId)
    </div>

    <div class="form-group">
        <button type="submit" class="btn btn-primary">Submit</button>
    </div>
}

@section scripts {
    <script src="~/lib/jquery-validation/dist/jquery.validate.js"></script>
    <script src="~/lib/jquery-validation-unobtrusive/jquery.validate.unobtrusive.js"></script>
    <script src="~/lib/js/example.js"></script>,
}

Answer 1

I had a similar situation but in my example I have a Root folder and depending on which root folder I am using the next drop down list would display the corresponding sub-folders.

Not sure if there is a purly asp.net solution but, I used Jquery/Ajax for this.

Your code should look something like this:

html list:

<label asp-for="StateId "></label>
<select asp-for="StateId " asp-items="Model.States" class="form-control" id="state-target"></select>
<span asp-validation-for="StateId " class="text-danger"></span>

<label asp-for="CityId"></label>
<select asp-for="CityId" asp-items="Model.Citys" class="form-control" id="city-target"></select>
<span asp-validation-for="CityId" class="text-danger"></span>

Jquery code, you write this in .js file and then add it to a specific view with this statement<script src="~/js/example.js"></script>, Don't forget you need to add a jquery library to your project before any other javascript, and your example.js will contain:

$(document).ready(function () {
  $("#state-target").on("change", function () {
    $list = $("#city-target");
    $.ajax({
        url: "/getCities",
        type: "GET",
        data: { id: $("#state-target").val() }, //id of the state which is used to extract cities
        traditional: true,
        success: function (result) {
            $list.empty();
            $.each(result, function (i, item) {
                $list.append('<option value="' + item["CityId"] + '"> ' + item["Name"] + ' </option>');
            });
        },
        error: function () {
            alert("Something went wrong call the police");
        }
    });
  });
});

The Ajax request will call this action in the Controller which will retrieve a list of cities from the database (using something like return dbContext.CityTable.Where(c => c.StateId == id).ToList() inside a getCititesFromDatabaseByStateId(id) method) and then return the Json object, the success function will create a list of options and apply it:

public IActionResult getCities(int id)
{
    var cities = new List<City>();
    cities = getCititesFromDatabaseByStateId(id); //call repository
    return Json(citites);
}

In your ViewModel consider changing IEnumerable<State/City/Zip> (IEnumerable<T>) to IEnumerable<SelectListItem>. I can say as well your Model's are messy (but if you can get data the from the database focus on getting the list working 1st), consider improving them later.

Fix for 2 errors mentioned in the comments:

public List<City> getCititesFromDatabaseByStateId(int id)
{
    return db.Citys.Where(c => c.StateId == id).ToList();
}

public ActionResult Create()
{
     var states = new SelectList(db.States.ToList(), "StateId", "Abbr");
     var citys = new SelectList(db.Citys.ToList(), "CityId", "Name");
     var zips = new SelectList(db.Zips.ToList(), "ZipId", "Code");

     var viewModel = new CustomerFormVM
     {
         States = states,
         Citys = citys,
         Zips = zips
     };

     return View(viewModel);
}