我有一些简单的代码可以从db中检索记录的ELMAH异常:
HealthMonitoringEntities context = new HealthMonitoringEntities();
IQueryable<ELMAH_Error> exceptions = context.ELMAH_Error;
if (filter.ToDate != null)
exceptions = exceptions.Where(e => e.TimeUtc <= filter.ToDate.Value.AddHours(-4));
return exceptions.OrderByDescending(e => e.TimeUtc)
.Take(filter.Size)
.AsEnumerable()
.Select(e => new ElmahException()
{
ErrorId = e.ErrorId,
Application = e.Application,
Host = e.Host,
Type = e.Type,
Source = e.Source,
Error = e.Message,
User = e.User,
Code = e.StatusCode,
TimeStamp = e.TimeUtc.AddHours(-4).ToString()
}).ToList();
}
我在这一行得到例外:
TimeStamp = e.TimeUtc.AddHours(-4).ToString()
例外是:
LINQ to Entities does not recognize the method 'System.DateTime AddHours(Double)' method, and this method cannot be translated into a store expression.
当我在使用.AsEnumerable()
进行投射之前调用Select()
时,我的序列会被枚举,并且我会从实现IEnumerable<ELMAH_Error>
的序列进行投影。鉴于此,为什么我不在我的投影中使用Linq-To-Objects API,它可以理解AddHours()
,而不是仍在使用Linq-To-Entities API?
更新
Jon Skeet在这个主题上发表了一篇文章:
他有这个问题:
var query = db.Context
.Customers
.Where(c => some filter for SQL)
.OrderBy(c => some ordering for SQL)
.Select(c => some projection for SQL)
.AsEnumerable() // Switch to "in-process" for rest of query
.Where(c => some extra LINQ to Objects filtering)
.Select(c => some extra LINQ to Objects projection);
请注意,在致电AsEnumerable()
后,他表示他正在切换到Linq-To-Objects。我在我的函数中做了类似的事情,但是我收到了一个Linq-To-Entities异常,我原以为我会针对Linq-To-Objects API执行。
进一步更新
来自Jim Wooley的博客:http://linqinaction.net/blogs/jwooley/archive/2009/01/21/linq-supported-data-types-and-functions.aspx
“作为示例,以下方法显示为具有DateTime值的翻译:Add,Equals,CompareTo,Date,Day,Month,Year。相比之下,不支持ToShortDateString,IsLeapYear,ToUniversalTime等方法。
如果您需要使用其中一种不受支持的方法,则需要将结果强制转换为客户端,并在此时使用LINQ to Objects对其进行评估。你可以在查询理解的任何一点使用.AsEnumerable扩展方法来做到这一点。“
这不是我正在做的吗?
答案 0 :(得分:0)
您忘记保护第一个电话
if (filter.ToDate != null)
exceptions = exceptions.AsEnumerable()
.Where(e => e.TimeUtc <= filter.ToDate.Value.AddHours(-4));
修改强>
请记住,IQueryables在枚举之前不会被转换为sql,因此您的调试器将执行该行,但在您返回之前不会发生错误。如果filter.ToDate
为空,则您的代码等效于此:
if(filter.ToDate == null)
return exceptions
.Where(e => e.TimeUtc <= filter.ToDate
.Value
.AddHours(-4)) //uh-oh won't work in Linq-to-Entities
.OrderByDescending(e => e.TimeUtc)
.Take(filter.Size)
.AsEnumerable() //too late to protect you!
.Select(e => new ElmahException()
{
ErrorId = e.ErrorId,
Application = e.Application,
Host = e.Host,
Type = e.Type,
Source = e.Source,
Error = e.Message,
User = e.User,
Code = e.StatusCode,
TimeStamp = e.TimeUtc.AddHours(-4).ToString() //if we get this far we're OK
}).ToList();