如何使用Entity Framework LINQ獲取列的SUM

Question

我想使用LINQ獲取“重量”列的SUM並存儲在列表中。 以下是兩列的聯接操作代碼。

var que = (from o in db.OrderPlans.Where(i => i.DemandId == _demandId)
join f in db.Fabrics
on o.FabricId equals f.Id
select new
{
    f.Id,
    f.Name,
    o.Width,
    f.Type,
    o.GSM,
    o.Weight
}).ToList();

Answer 1

假設您上面發布的代碼有效，並且假設您將其存儲在一個名為joinedList的變量中（並且您希望將由所有其他字段的不同組合形成的組的權重之和）：

var groupedList = joinedList.GroupBy(grp => new {
                             grp.Id,
                             grp.Name,
                             grp.Width,
                             grp.Type,
                             grp.GSM
}).Select(grp => new {
                             grp.Key.Id,
                             grp.Key.Name,
                             grp.Key.Width,
                             grp.Key.Type,
                             grp.Key.GSM,
                             SumOfWeight = grp.Sum(w => w.Weight)
}).ToList();

Answer 2

在您的代碼末尾附加此.Groupby(w => w.Weight).Convert< int >(g=> g.Sum());

Answer 3

在我看來， Fabric和OrderPlan之間存在一對多的關系：每個Fabric具有零個或多個OrderPlans ，每個OrderPlan完全屬於一個Fabric ，即外鍵引用的Fabric 。

在我看來，您想要具有其OrderPlans （的幾個屬性）和這些OrderPlans的總權重的幾個Fabric屬性。

每當您具有一對多關系或多對多關系並且想要獲取“帶有子項的項目”時，例如學校與學生，客戶與訂單或訂單與訂單行，例如使用Queryable.GroupJoin而不是標准聯接 。

// GroupJoin Fabrics with some of their OrderPlans:
var result = dbContext.Fabrics
    .GroupJoin(dbContext.OrderPlans.Where(orderPlan => ...)
    fabric => fabric.Id,              // from each Fabric take the primary key
    orderPlan => orderPlan.FabricId,  // from each OrderPlan take the foreign key

    // ResultSelector: take every Fabric with all his matching OrderPlans to make one new object:
    (fabric, orderPlansOfthisFabric) => new
    {
        // Select the Fabric properties you want
        Id = fabric.Id,
        Name = fabric.Name,
        Type = fabric.Type,

        OrderPlans = orderPlansOfThisFabric.Select(orderPlan => new
        {
             // Select the OrderPlan properties you want:
             Width = orderPlan.Width,
             Gsm = orderPlan.Gsm,
        }

        // Weight: calculate the sum of all OrderPlans of this Fabric:
        Weight = orderPlansOfThisFabric
                 .Select(orderPlan => orderPlan.Weight)
                 .Sum(),
    });