My form ignores Required attributes from model - asp.net

Question

I'm having troubles with my form validation. For some reason the form ignores any [Required] attribute I have in my model.
Model: (Stripped down for easy read)
public class NotEmployeesModel
{
public NotEmployeesModel()
{
DetailModel = new NotEmployeesDetailModel();
}
public NotEmployeesDetailModel DetailModel { get; set; }
}
public class NotEmployeesDetailModel
{
public NotEmployeesDetailModel()
{
}
public NotEmployeesDocumentModel DocumentModel { get; set; }
}
public class NotEmployeesDocumentModel
{
public NotEmployeesDocumentModel()
{
}
public NotEmployeeDocumentInputModel DocumentInput { get; set; }
public class NotEmployeeDocumentInputModel
{
public NotEmployeeDocumentInputModel()
{
}
public NotEmployeeDocumentInputModel(int notEmployeeId)
{
NotEmployeeId = notEmployeeId;
}
public int NotEmployeeId { get; set; }
public int SelectedDocumentType { get; set; }
[Required(ErrorMessageResourceType = typeof(ErrorMessages), ErrorMessageResourceName = "Star")]
public string Description { get; set; }
[Required(ErrorMessageResourceType = typeof(ErrorMessages), ErrorMessageResourceName = "Star")]
public HttpPostedFileBase File { get; set; }
}
}
For each view or partial I have a separate model class.
Form:
#model NotEmployeesDocumentModel
#using (Html.BeginForm("AddNotEmployeeDocument", "Home", FormMethod.Post, new { id = "form-add-document", enctype = "multipart/form-data" }))
{
#Html.HiddenFor(x => x.DocumentInput.NotEmployeeId)
<table class="table-output">
<thead>
<tr>
<td>#Html.Label("Sort", Labels.Sort)</td>
<td>#Html.Label("Description", Labels.Description)</td>
<td>#Html.Label("Type", Labels.Type)</td>
<td class="text-align-right"></td>
</tr>
</thead>
<tbody>
<tr>
<td>
<i class="fa fa-plus-square cursor-pointer add-document"></i>
<input type="file" id="DocumentInput_File" name="DocumentInput.File" required="required" />
#Html.ValidationMessageFor(x => x.DocumentInput.File)
</td>
<td>
#Html.TextBoxFor(x => x.DocumentInput.Description, new { #class = "width300px hardware-setup-input", placeholder = "Vul hier een omschrijving in..." })
#Html.ValidationMessageFor(x => x.DocumentInput.Description)
</td>
<td>
#Html.DropDownListFor(x => x.DocumentInput.SelectedDocumentType, Model.DocumentTypes, "--- Maak een keuze ---")
</td>
<td class="text-align-right">
<span id="btn-add-document" class="button-org">#Labels.Save</span>
</td>
</tr>
</tbody>
</table>
}
Structure of my page: View / Partial / Partial (Here is my form)
JS:
$("#btn-add-document").on("click", function () {
var frm = $("#form-add-document");
if (frm.valid()) {
frm.ajaxSubmit({
dataType: "html",
success: function (responseText) {
$("#document-container").html(responseText);
$("#DocumentInput_Description").text("");
$("#DocumentInput_SelectedDocumentType").val("");
loadDocumentPartial();
}
});
}
});
I'm using jQuery Form plugin from malsup to submit my form through ajax.
Controller:
[HttpPost]
public ActionResult AddNotEmployeeDocument(NotEmployeesDocumentModel input)
{
// do some code
}
As you can see in the parameters of my ActionResult I can't put NotEmployeesDocumentInputModel like I always do but I'm forced to use the parent class.
I have no idea what I'm doing wrong. This is the first time I encounter such problem.