带有SUM 和 Group by 的SQL 在 Lambda 的写法

例如：

SELECT partName,sum(partWeight) as totalWeight
FROM tblParts
GROUP BY partName

Linq to sql 写法：

var sr = from k in db.tblParts
         group by k.partName into g
         select new
         {
            partName = g.Key,
            totalWeight = g.Sum(k => k.partWeight)
         }