通过EntityFramework获取最新的表格值 [英] Get the latest Value of table via EntityFramework

问题描述

我有一个表格有几个字段，每个表单由ViewModel单独更新，现在我想获取一个特定字段的最新值（也许它已经在我的表的第五条记录中更新），OK？现在我写的是这样的：

  public ViewResult ShowPiece（）
 {
 var context = new SiteContext（）; 
 var showPiece = context.Portraits.LastOrDefault（）; 
 return View（showPiece）; 
} 
  

但是当我运行应用程序并浏览以上操作时，我遇到了错误：

LINQ to Entities不识别该方法，并且此方法无法转换为存储表达式...

这是什么问题？

解决方案

使用降序（按日期或id）和 FirstOrDefault 这是支持的：

  var showPiece = context.Portraits 
 .OrderByDescending（p => p.Date）
 .FirstOrDefault （）; 
  

---

另一个选项是选择最大日期的肖像（id）通过子查询（如Evelie在评论中提出的）：

  var showPiece = context.Portraits 
 .FirstOrDefault p => p.Date == context.Portraits.Max（x => x.Date））; 
  

我在这里做了一些调查。在第一种情况下（顺序）生成以下查询：

  SELECT TOP（1）[t0]。* 
 FROM [肖像] AS [t0] 
 ORDER BY [t0]。[日期] DESC 
  

在第二种情况下（获得最大值）：

  SELECT TOP（1）[t0]。* 
 FROM [Portraits] AS [t0] 
 WHERE [t0]。[Date] =（（
 SELECT MAX（[t1]。[Date]）
 FROM [Portraits] AS [t1] 
 ））
  

执行计划几乎相同，但在第二种情况下，Top被执行两次。因此，与索引扫描相比，顶部成本为0％，这不应该是一个问题。

I have a table that have several field and each of them update separately by separate ViewModel , Now I wanna to get the latest Value of a specific field (maybe it has updated in fifth record of my table) , OK? now what I have write is look like this :

  public ViewResult ShowPiece()
        {
            var context = new SiteContext();
            var showPiece = context.Portraits.LastOrDefault();
            return View(showPiece);
        }

but when I run the application and navigate above action , I got thie Error :

LINQ to Entities does not recognize the method , and this method cannot be translated into a store expression...

what is the problem with that ??

解决方案

Use descending ordering (by date, or id) and FirstOrDefault which is supported:

var showPiece = context.Portraits
                       .OrderByDescending(p => p.Date)
                       .FirstOrDefault();

---

Another option, is select portrait which has max date (id) via subquery (as Evelie suggested in comments):

var showPiece = context.Portraits
              .FirstOrDefault(p => p.Date == context.Portraits.Max(x => x.Date));

I made a little investigation here. In first case (ordering) following query is generated:

SELECT TOP (1) [t0].*
FROM [Portraits] AS [t0]
ORDER BY [t0].[Date] DESC

In second case (getting max):

SELECT TOP (1) [t0].*
FROM [Portraits] AS [t0]
WHERE [t0].[Date] = ((
    SELECT MAX([t1].[Date])
    FROM [Portraits] AS [t1]
    ))

Execution plan is almost same, but in second case Top is executed twice. Thus Top costs 0% comparing to Index Scan, this should not be a problem.

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