实体框架查询选择新的连接答案

【问题标题】：entity framework query select new concat实体框架查询选择新的连接
【发布时间】：2015-04-16 20:35:18
【问题描述】：

我怎么能做这样的事情？我认为问题在于“选择”中的连接。

var respaldos = db.Respaldoes.Where(x => x.type == "Backup")
    .Where(x => x.start_time >= _desde && x.start_time <= _hasta)
    .GroupBy(x => new
    {
        x.start_time.Year,
        x.start_time.Month,
        x.start_time.Day
    })
    .Select(x => new
    {
        periodo = x.Key.Day + "/" + x.Key.Month + "/" + x.Key.Year,
        objetos = x.Sum(y => y.bytes_processed)
    });

foreach (var obj in respaldos)
{
    Dictionary<string, string> dicciconario = new Dictionary<string, string>();
    dicciconario.Add("periodo", obj.periodo.ToString());
    dicciconario.Add("objetos", obj.objetos.ToString());
}

运行应用程序时出现错误，错误在“foreach”中

错误：没有 se pudo convertir el tipo 'System.Int32' al tipo 'System.Object'。 LINQ to Entities sólo admissione la conversión a tipos de enumeración o primitivos de EDM。

我尝试一下：

.Select(x => new
{
    periodo = x.Key.Day.ToString() + "/" + x.Key.Month.ToString() + "/" + x.Key.Year.ToString(),
    objetos = x.Sum(y => y.bytes_processed)
});

我收到其他错误：

LINQ to Entities no reconoce el método System.String ToString() del método, y este método no se puede traducir en una expresión de almacén。

【问题讨论】：

请用翻译成英文的替换原来的错误信息...
我会改变它：.GroupBy(x => new{ x.start_time.Year, x.start_time.Month, x.start_time.Day }).Select(...); 到 GroupBy(x => x.start_time).Select(grp=>new{periodo = grp.Key, objectos = grp.Sum(y=>y.bytes_processed)});
How to format a string in a linq expression? 的可能重复项
翻译：Error: Could not convert type 'System.Int32' to type 'System.Object'. LINQ to Entities only supports conversion Enumeration or primitive types of EDM. 第二个错误：LINQ to Entities does not recognize the System.String ToString () method of the method, and this method can not be translated into a store expression.

标签： c# linq entity-framework asp.net-mvc-4 concat

【解决方案1】：

您似乎想按没有时间组件的日期进行分组。 EF 有一个功能：DbFunctions.TruncateTime:

var respaldos = db.Respaldoes.Where(x => x.type == "Backup")
    .Where(x => x.start_time >= _desde && x.start_time <= _hasta)
    .GroupBy(x => DbFunctions.TruncateTime(x.start_time))
    .Select(x => new
    {
        periodo = x.Key, // Now a DateTime value
        objetos = x.Sum(y => y.bytes_processed)
    });

在这之后，你可以做

Dictionary<DateTime, int> dicciconario =
   respaldos.ToDictionary(x => x.periodo, x => x.objetos);

【讨论】：