Question

int year = 2009; // get summ of TONS2009 column

var query = from ODInfo in DataContext.CIMS_TRUCKS
            where pLocationIDs.Contains(ODInfo.OID) 
            group ODInfo by ODInfo.OID into g
            select new
            {
               OID = g.Key,
               TotalTons = g.Sum( ODInfo => ODInfo.TONS2009)
            };

表达式'ODInfo =＆gt; ODInfo.TONS2009'，如何根据方法参数'int year'将TONS2009更改为TONS2010或TONS2011？

Answer 1

K06a的答案很接近，但在服务器端不起作用。试试这个：

IEnumerable<OutputType> myQuery(IEnumerable<InputType> data, Expression<Func<InputType,decimal>> expr)
{
    return from ODInfo in DataContext.CIMS_TRUCKS
           where pLocationIDs.Contains(ODInfo.OID) 
           group ODInfo by ODInfo.OID into g
           select new OutputType
          {
              OID = g.Key,
              TotalTons = g.AsQueryable().Sum(expr)
           };
}

var query = myQuery(DataContext.CIMS_TRUCKS, ODInfo => ODInfo.TONS2009);

我没有试过这个，但做了类似here的事情。

<强>更新

如果你真的需要将输入字符串（如“2009”）翻译成表达式，它仍然可能：

string year = "2009";

Type ODInfoType = typeof(ODINFOTYPE); // substitute with the type of ODInfo
ParameterExpression pe = ParameterExpression.Parameter(ODInfoType, "ODInfo");
MemberInfo mi = ODInfoType.GetProperty("TONS" + year);
MemberExpression me = Expression.MakeMemberAccess(pe, mi);
var expr = Expression.Lambda<Func<ODINFOTYPE, decimal>>(me, pe);

请注意，这是对数据库极端邪恶结构的补丁。

Answer 2

最好的解决方案是将其分解为多个查询，您可以撰写最终查询：

int year = 2009; // get summ of TONS2009 column

var odInfos =
   year == 2009 ? DataContext.CIMS_TRUCKS.Select(x => new { x.OID, TONS = x.TONS2009 })
   year == 2010 ? DataContext.CIMS_TRUCKS.Select(x => new { x.OID, TONS = x.TONS2010 })
   year == 2011 ? DataContext.CIMS_TRUCKS.Select(x => new { x.OID, TONS = x.TONS2011 })
   : null;

var query = from ODInfo in odInfos
            where pLocationIDs.Contains(ODInfo.OID) 
            group ODInfo by ODInfo.OID into g
            select new
            {
               OID = g.Key,
               TotalTons = g.Sum(ODInfo => ODInfo.TONS)
            };

这将专门针对运行时的三个可能的查询，从而提供最佳性能。它比案例转换更好。

Answer 3

尝试这种方式：

IEnumerable<OutputType> myQuery(IEnumerable<InputType> data, Func<InputType,decimal> func)
{
    return from ODInfo in data
           where pLocationIDs.Contains(ODInfo.OID) 
           group ODInfo by ODInfo.OID into g
           select new OutputType
           {
              OID = g.Key,
              TotalTons = g.Sum(func)
           };
}

var query = myQuery(DataContext.CIMS_TRUCKS, ODInfo => ODInfo.TONS2009);

Answer 4

使用适用于EF的DynamicLinq：

int year = 2009; // get summ of TONS2009 column

var query = from ODInfo in DataContext.CIMS_TRUCKS
            where pLocationIDs.Contains(ODInfo.OID) 
            group ODInfo by ODInfo.OID into g
            select g;

var projectedGroups = query.Select("new (Key as OID, Sum(TONS" + year + ") as TotalTons)");

动态更改LINQ to Entity查询

4 个答案: