比較兩個表ASP.Net MVC的ID

Question

我有兩張桌子。

作者表--- AuthID PK

合同表--- AuthID FK

我想將合同視圖“作者姓名”框限制為尚未擁有合同的作者。

我想我可以用更多的代碼做到這一點，但我覺得有一種方法可以通過一個簡單的查詢來實現它，我只是錯過了它。

這是我的代碼：

 public ActionResult Create()
    {
        var contract = new Contract();

        var allAuthors = db.Authors.Select(a => a.AuthID);
        var unusedAuthors = new List<Author>();
        foreach (var auth in allAuthors) {
            unusedAuthors = db.Contracts
                .Where(a => a.AuthID.GetHashCode() != auth.GetHashCode())
                .Select(a => a.Author).ToList();
        }




        ViewBag.AuthID = new SelectList(unusedAuthors, "AuthID", "AuthFirstName");
        return View(contract);
    }

Answer 1

你可以試試這個：

 var unusedAuthors =
    (from a in db.Authors
    join c in db.Contracts on a.AuthID equals c.AuthID into lrs
    from lr in lrs.DefaultIfEmpty()
    where lr ==null
    select  a).ToList() ;

Answer 2

你可以這樣做

var allAuthors = db.Authors.Select(a => a.AuthID).ToList();


var unusedAuthors = db.Contracts
                .Where(x => !(allAuthors.Contains(x.AuthID)))
                .Select(a => a.Author)
                .ToList();

不需要使用foreach