必需属性不适用于外键下拉列表

我使用数据库优先方法创建了一个项目，其中我的表

城市-包含以下列

City_Id
City_Name
State_Id

State_Id是状态表的外键。

现在，所有验证和所有操作均正常进行，直到我在City.cs模型文件中的State_Id上放置[Required]批注，这是

    //------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated from a template.
//
//     Manual changes to this file may cause unexpected behavior in your application.
//     Manual changes to this file will be overwritten if the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------
namespace SS.Models
{
    using System;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.ComponentModel.DataAnnotations;
    using System.Web.Mvc;
    public partial class City
    {
        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
        public City()
        {
            this.Areas = new HashSet<Area>();
        }

        public int City_Id { get; set; }

        [Required]
        [DisplayName("City Name")]
        [StringLength(50,ErrorMessage = "City Name exceeds limit of 50 characters")]
        [RegularExpression(@"[A-Z][a-z]+",ErrorMessage ="Input Format Incorrect")]
        [Remote("CheckDuplicateInState","Cities",AdditionalFields ="State_Id",ErrorMessage ="There cannot be duplicate city names in same state")]
        public string City_Name { get; set; }


        [DisplayName("State Name")]
        public int State_Id { get; set; }

        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
        public virtual ICollection<Area> Areas { get; set; }

        public virtual State State { get; set; }
    }
}

当我在State_Id上放置[Required]并在Create视图中的表单中按下按钮Submit时，什么都不会发生，即使我从下拉列表中选择了正确的状态也不会发生回发，或者我只是选择了默认的显示值“ Select”状态”，则也不会显示任何错误。

我的看法如下

@model SS.Models.City

@{
    ViewBag.Title = "Create";
}

<h2>Create</h2>


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

    <div class="form-horizontal">
        <h4>City</h4>
        <hr />
        @Html.ValidationSummary(true, "", new { @class = "text-danger" })
        <div class="form-group">
            @Html.LabelFor(model => model.City_Name, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.City_Name, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.City_Name, "", new { @class = "text-danger" })
            </div>
        </div>

        <div class="form-group">
            @Html.LabelFor(model => model.State_Id,htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.DropDownList("State_Id", null,"Select State", htmlAttributes: new { @class = "form-control" })
                @Html.ValidationMessageFor(model => model.State_Id, "", new { @class = "text-danger" })
            </div>
        </div>

        <div class="form-group">
            <div class="col-md-offset-2 col-md-10">
                <input type="submit" value="Create" class="btn btn-default" />
            </div>
        </div>
    </div>
}

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

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

我的控制器（相关代码）如下

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Entity;
using System.Linq;
using System.Net;
using System.Web;
using System.Web.Mvc;
using SS.Models;

namespace SS.Controllers.Admin
{
    public class CitiesController : Controller
    {
        private SS db = new SSDatabaseEntities();

        // GET: Cities/Create
        public ActionResult Create()
        {
            ViewBag.State_Id = new SelectList(db.States, "State_Id", "State_Name");
            return View();
        }

        // POST: Cities/Create
        // To protect from overposting attacks, please enable the specific properties you want to bind to, for 
        // more details see http://go.microsoft.com/fwlink/?LinkId=317598.
        [HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult Create([Bind(Include = "City_Id,City_Name,State_Id")] City city)
        {
            if (ModelState.IsValid)
            {
                db.Cities.Add(city);
                db.SaveChanges();
                return RedirectToAction("Index");
            }

            return View(city);
        }



        public JsonResult CheckDuplicateInState(string City_Name,int state_id)
        {
            State st = db.States.Find(state_id);

            if (st.Cities.Any(x => x.City_Name == City_Name))
                return Json(false, JsonRequestBehavior.AllowGet);
            else
                return Json(true, JsonRequestBehavior.AllowGet);
        }
    }
}

将[Required]放在City.cs的State_Id上时，在查看页面源代码的下拉html中不会生成data-val-required的必需属性。

    <div class="form-group">
            <label class="control-label col-md-2" for="State_Id">State Name</label>
            <div class="col-md-10">
                <select class="form-control" id="State_Id" name="State_Id"><option value="">Select State</option>
<option value="1">Madhya Pradesh</option>
<option value="2">Gujarat</option>
<option value="4">Punjab</option>
<option value="9">Tamil Nadu</option>
<option value="10">Haryana</option>
</select>
                <span class="field-validation-valid text-danger" data-valmsg-for="State_Id" data-valmsg-replace="true"></span>
            </div>
        </div>