MultiSelectList validation for numeric id value - asp.net-mvc

Question

I have a MultiSelectList whose dataValueField is a number in a code-behind model and dataTextField field is string.
When I select multiple values in the resulting html select element I get validation error saying that the field must be a number. This makes sense because the backing field is an integer and when you select several entries the id values for buildings are concatenated using commas. What would be the work around for this? Thanks.
The model is as follows.
// Selected buildings are stored in this table.
public class ClientSelectedBuildings
{
public int ClientSelectedBuildingsId { get; set; }
// ...
[Display(Name = "Please select the buildings under consideration.")]
public int? BuildingId { get; set; }
}
// Building list is retrieved from this table.
public class Buildings
{
public int BuildingsId { get; set; }
// ...
[StringLength(255)]
public string BuildingName { get; set; }
}
My view looks as follows.
#model TheApplication.Models.ClientSelectedBuildings
<div class="outer">
<div class="inner">
#Html.LabelFor(t => t.BuildingId)
#Html.ListBoxFor(t => t.BuildingId, (MultiSelectList)ViewBag.Buildings, new { size = "4" })
#Html.ValidationMessageFor(t => t.BuildingId)
</div>
</div>

Answer 1

The issue is that your domain model is only allowing one BuildingId, yet the form will attempt to send multiple via the list box.
This is a perfect example where your domain model does not perfectly match up with the view model. Domain and View each have different concerns and outside of very very basic CRUD situations, you'll find that form views will always require a separate model.
You won't be able to bind directly to ClientSelectedBuildings (without a custom modelbinder). Instead, bind to an intermediate model that then can be mapped into multiple ClientSelectedBuildings.
// Here's the model submitted from the view. This will have to be mapped to domain
// entities.
public class FormModel
{
// ... Other form fields ...
public int[] BuildingIds { get; set;
// ... Other form fields ...
}
// Example controller action that processes the form input.
[HttpPost]
public ActionResult MyPostAction(FormModel input)
{
if (ModelState.IsValid)
{
// Loop all submitted building ids, map the data into domain entities.
foreach(var buildingId in input.BuildingIds)
{
// Create the domain entity.
var selected = new ClientSelectedBuildings
{
ClientSelectedBuildingsId = ... Wherever this comes from ...
BuildingId = buildingId;
};
// Add to the data repository.
this.MyDbContext.ClientSelectedBuildings.Add(selected);
}
// Submit all changes to the data context.
this.MyDbContext.ClientSelectedBuildings.SaveChanges();
// ... Return redirect to success view ...
}
// ... Return error view ...
}