为什么我的外键没有设置？

Question

我正在使用Entity Framework，代码优先，我在控制器中有这个代码

_auctionContext.AddToAuctionEvents(new AuctionEvent
    {
        Title = model.Title,
        Description = model.Description,
        Starts = model.Starts,
        Ends = model.Ends,
        DepositAmount = model.DepositAmount,
        Currency = _auctionContext.Currency.Where(c => c.Id == model.Currency).First()
    });
_auctionContext.SaveChanges();

我认为我有这个

<section>
    @Html.LabelFor(model => model.Currencies)
    <div>
        @Html.DropDownList("Currency", Model.Currencies, "Select Currency", new { required = "required" }) 
        @Html.ValidationMessageFor(model => model.Currencies)
    </div>
</section>

上面提到的我的viewmodel看起来像这样

public class AuctionEventDetailsModel
{
    public AuctionEventDetailsModel()
    {
        Currencies = new List<SelectListItem>();
    }

    public string Title { get; set; }
    public string Description { get; set; }
    public DateTime Starts { get; set; }
    public DateTime Ends { get; set; }
    public decimal DepositAmount { get; set; }
    public List<SelectListItem> Currencies { get; set; }
    public int Currency { get; set; }
}

由于某些原因，当控制器中的代码执行时，它总是在数据库中Currency_Id表的AuctionEvents字段中放置一个空值。这是为什么？