Question

我有一个控制器，如果特定数据不存在，它将创建一个新行，否则它将返回数据已存在的警报。问题是我有一个查询，即使我的表是空的，它总是说不为空。

控制器：

 public ActionResult CreateDisease(int? diseaseID, int? assessmentID, DiseaseList diseaselist)
    {
        var data = from x in db.DiseaseLists where ((x.AssessmentID == assessmentID) && (x.DiseaseID == diseaseID)) select x;

        if (data == null)
        {
            diseaselist.DiseaseID = diseaseID;
            diseaselist.AssessmentID = assessmentID;
            db.DiseaseLists.Add(diseaselist);
            db.SaveChanges();
            return Json("Success", JsonRequestBehavior.AllowGet);
        }
        else
        {
            return Json("Already Exist", JsonRequestBehavior.AllowGet);
        }

    }

Answer 1

您的数据不会是null，它只会是一个空列表。将代码更改为此以检查是否找到了您要查找的对象。

public ActionResult CreateDisease(int? diseaseID, int? assessmentID, DiseaseList diseaselist)
    {
        var data = from x in db.DiseaseLists where ((x.AssessmentID == assessmentID) && (x.DiseaseID == diseaseID)) select x;

        if (!data.Any())
        {
            diseaselist.DiseaseID = diseaseID;
            diseaselist.AssessmentID = assessmentID;
            db.DiseaseLists.Add(diseaselist);
            db.SaveChanges();
            return Json("Success", JsonRequestBehavior.AllowGet);
        }
        else
        {
            return Json("Already Exist", JsonRequestBehavior.AllowGet);
        }

    }

Answer 2

正如Stephen Muecke在上面的评论中所说，它永远不会是空的，只是空的，这就是为什么它不断返回一个对象，所以我尝试了CodeCaster的建议并且它完美无缺。

而不是

var data = from x in db.DiseaseLists where ((x.AssessmentID == assessmentID) && (x.DiseaseID == diseaseID)) select x;

我试过

var data = db.DiseaseLists.Where(x => (x.AssessmentID == assessmentID) && (x.DiseaseID == diseaseID)).SingleOrDefault();

它完美无缺。

始终返回不为空

2 个答案: