C# 如何使用实体框架LINQ获取列和

我想使用LINQ获得列“Weight”的和，并存储在列表中。以下是两列联接操作的代码

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();

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假设您上面发布的代码有效，让我们假设您将其存储在一个名为joinedList的变量中（并且您希望得到由所有其他字段的不同组合组成的组的权重之和）：

---

在你的代码末尾附加这个

---

.Groupby（w=>w.Weight）.Convert（g=>g.Sum（））

在我看来，

结构

和

订单计划

之间有一对多的关系：每个

结构

都有零个或多个

订单计划

，每个

订单计划

只属于一个

结构

，即外键所指的

结构

在我看来，您需要几个

Fabric

属性及其

OrderPlans

的（几个属性）以及这些

OrderPlans

的总重量

当您有一对多关系或多对多关系，并且希望获取“项目及其子项目”，例如学校及其学生、客户及其订单或订单及其订单行时，您可以使用而不是标准联接

// 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(),
    });

---

您需要先应用group by，然后才能使用Sum运算符。@sriharsha您能对该代码进行注释吗？

// 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(),
    });