Linq Joins 仅在所有表都有数据时才起作用

Question

您可以在下面找到我的问题。当“DocentenCompetenties”和“DocentenLocaties”都被填写时，我的视图只会循环遍历数据。这是一个问题，因为即使其中一个没有任何数据，我也希望能够循环它们

var ShowCompetenties = from d in db.Docent
      join dc in db.DocentenCompetenties on d.DocentID equals dc.DocentID
      join c in db.Competenties on dc.CompetentiesID equals c.CompetentiesID
      join dl in db.DocentenLocaties on d.DocentID equals dl.DocentID
      where d.DocentID == id
      join l in db.Locaties on dl.LocatieID equals l.LocatieID
      select new ShowCompetenties { Docenten = d, Competenties = c, DocentenCompetenties = dc, DocentenLocaties = dl, Locaties = l };

更新

当前错误： App_Web_xp01otas.dll 中出现“System.NullReferenceException”类型的异常，但未在用户代码中处理 附加信息：对象引用未设置为对象的实例。

    var id = Convert.ToInt32(Session["id"]);
    var LeftShowCompetenties = from d in db.Docent
       join g1 in db.DocentenCompetenties on d.DocentID equals g1.DocentID into group1
       from dc in group1.DefaultIfEmpty()                                    
       join c in db.Competenties on dc.CompetentiesID equals c.CompetentiesID
       where d.DocentID == id                          
       select new ShowCompetenties { Docenten = d, Competenties = c, DocentenCompetenties = dc};


    var RightShowCompetenties = from d in db.Docent                                         
        join g3 in db.DocentenLocaties on d.DocentID equals g3.DocentID into group3      from dl in group3.DefaultIfEmpty()
        where d.DocentID == id
        join l in db.Locaties on dl.LocatieID equals l.LocatieID
        select new ShowCompetenties { Docenten = d, Locaties = l, DocentenLocaties = dl };



    var ShowCompetenties = LeftShowCompetenties.Union(RightShowCompetenties);

查看

<h4>Competenties</h4>
@foreach (var item in Model)
{
            if(@item.DocentenCompetenties != null && @item.DocentenCompetenties.DocentID.ToString() != null) { 
            @item.Competenties.Name @Html.ActionLink("Delete", "DeleteCompetenties", new { id = item.DocentenCompetenties.DocentenCompetentiesID })
}

 <h4>Docenten</h4>
 @foreach (var item in Model)
 {
      if(@item.DocentenLocaties != null && @item.DocentenLocaties .DocentID.ToString() != null) 
        {
            @item.Locaties.Name
        }
}

Answer 1

您正在以这种方式进行内部联接。

听起来您实际上想要进行外部联接。 试试这个：

var ShowCompetenties = from d in db.Docent
  join g1 in db.DocentenCompetenties on d.DocentID equals g1.DocentID into group1
  from dc in group1.DefaultIfEmpty()

  join g2 in db.Competenties on dc.CompetentiesID equals g2.CompetentiesID into group2
  from c in group2.DefaultIfEmpty()

  join g3 in db.DocentenLocaties on d.DocentID equals g3.DocentID into group3
  from dl in group3.DefaultIfEmpty()

  where d.DocentID == id
  join l in db.Locaties on dl.LocatieID equals l.LocatieID
  select new ShowCompetenties { Docenten = d, Competenties = c, DocentenCompetenties = dc, DocentenLocaties = dl, Locaties = l };

但是，如果您希望进行完全外连接，这只是一个左外连接。您必须先进行左外连接，然后进行右外连接，最后合并（与Union()）两者。

编辑 根据 cmets，关于您在 UNION 之后遇到的错误：

    var id = Convert.ToInt32(Session["id"]);
        var LeftShowCompetenties = from d in db.Docent
            join g1 in db.DocentenCompetenties on d.DocentID equals g1.DocentID into group1
            from dc in group1.DefaultIfEmpty()                                    
            join c in db.Competenties on dc.CompetentiesID equals c.CompetentiesID
            where d.DocentID == id                          
            select new ShowCompetenties { Docenten = d, Competenties = c, Locaties = null, DocentenCompetenties = dc, DocentenLocaties = null};


    var RightShowCompetenties = from d in db.Docent                                         
           join g3 in db.DocentenLocaties on d.DocentID equals g3.DocentID into group3
           from dl in group3.DefaultIfEmpty()
           where d.DocentID == id
           join l in db.Locaties on dl.LocatieID equals l.LocatieID
           select new ShowCompetenties { Docenten = d, Competenties = null, Locaties = l, DocentenCompetenties = null, DocentenLocaties = dl };



   var ShowCompetenties = LeftShowCompetenties.Union(RightShowCompetenties);

（检查构造函数中添加的Locaties = null和Competenties = null。）

查看

if (@item.DocentenCompetenties != null){}
if (@item.DocentenLocaties!= null){}

Answer 2

如果您设置了导航属性，则左连接会容易得多：

var ShowCompetenties =
  from d in db.Docent
  where d.DocentID == id
  from dc in d.DocentenCompetenties.DefaultIfEmpty()
  let c = dc.Competenty
  from dl in d.DocentenLocaties.DefaultIfEmpty()
  let l = dl.Locaty
  select new ShowCompetenties {
    Docenten = d,
    Competenties = c,
    DocentenCompetenties = dc,
    DocentenLocaties = dl,
    Locaties = l };