投影到模型中时，将COUNT（）分配给模型字段

Question

我有2个表希望加入其中并从中建立模型： Issues和IssueAttachments 。 模型中的字段之一是每个Issue的所有IssueAttachments的计数。 在SQL中，很简单：

SELECT i.*, 
       ISNULL
       (
           (Select COUNT(IssueID) FROM IssueAttachments WHERE IssueID = i.IssueID),
           0
       ) AS 'Number of Attachments'
FROM Issues

但是我很难将其转换为linq。 这是我的声明：

var issues = from i in db.Issues
    join ia in db.IssueAttachments
    on i.IssueID equals ia.IssueID into issAttachments
    from issueAttachments in issAttachments.DefaultIfEmpty()
    select new IssueModel
    {
      IssueID = i.IssueID,
      /*** more fields ***/
      NumberOfAttachments = ???
    }

??? 在这里我需要每个问题的IssueAttachments数量。 我尝试了这个：

(from ia in db.IssueAttachments
 where ia.IssueID == i.IssueID
 select ia).Count()

但是我Unable to create a constant value of type错误Unable to create a constant value of type 。 我在SO周围寻找了一些示例，但是由于缺乏对linq的了解，我不得不在大多数情况下挠头。

任何帮助是极大的赞赏。

Answer 1

您可以尝试以下一种方法：

// Initially, we make a group of all the issue attachments based on the IssueID
// and we count the corresponding number of attachments.

var attachments = db.IssueAttachments
                    .GroupBy(x=>x.IssueID)
                    .Select(x=> new { IssueID = x.Key, 
                                      NumberOfAttachments = x.Count()
                    });

然后

// Later we join the Issues tables rows with attachments results based on the IssueID
// and we select the corresponding data
var issues = from i in db.Issues
             join a in attachments
             on i.IssueID equals a.IssueID
             select new IssueModel
             {
                 IssueID = i.IssueID,
                 /*** more fields ***/
                 NumberOfAttachments = a.NumberOfAttachments
             };

注意

如果您需要在更多字段中获取附件的值，则必须首先按照我们在组中进行选择。 请让我知道，以便更新我的帖子。 谢谢。

Answer 2

var result = db.Issues
      .GroupJoin
      (
          db.IssueAttachments ,
          x=>x.IssueID ,
          x=>x.IssueID ,
          (i,ia)=>new
          {
              i.IssueID,
              /*** more fields ***/
              NumberOfAttachments = ia.Count()
          }
      );