这是对此的回应question https://stackoverflow.com/questions/315966/how-do-you-construct-a-linq-to-entities-query-to-load-child-objects-directly-ins/550118#550118在另一个问题的答案部分。
我有一个 Orders 集合,每个 Order 都是 OrderItems 的集合,每个 OrderItem 都有一个 PartId。如何使用 LINQ 实现以下 SQL 语句:
1) 选择具有特定零件 ID 的所有订单
SELECT *
FROM Order
WHERE Id in (SELECT OrderId FROM OrderItems WHERE PartId = 100)
2) 选择Order.OrderNumber和OrderItem.PartName
SELECT Order.OrderNumber, OrderItem.PartName
FROM Order INNER JOIN OrderItem ON Order.Id = OrderItem.OrderId
WHERE OrderItem.partId = 100
3) 选择 Order.OrderNumber 和整个 OrderItem 详细信息:
SELECT Order.OrderNumber, OrderItem.*
FROM Order INNER JOIN OrderItem ON Order.Id = OrderItem.OrderId
WHERE OrderItem.partId = 100
实际代码应该是
1)
var orders = from o in Orders
where o.OrderItems.Any(i => i.PartId == 100)
select o;
Any() 方法返回一个布尔值,类似于 SQL 的“in”子句。这将获取包含 PartId 为 100 的任何 OrderItems 的所有订单。
2a)
// This will create a new type with the 2 details required
var orderItemDetail = from o in Orders
from i in Orders.OrderItems
where i.PartId == 100
select new()
{
o.OrderNumber,
i.PartName
}
两个 from 子句就像一个内连接。
2b)
// This will populate the OrderItemSummary type
var orderItemDetail = from o in Orders
from i in Orders.OrderItems
where i.PartId == 100
select new OrderItemSummary()
{
OriginalOrderNumber = o.OrderNumber,
PartName = i.PartName
}
3)
// This will create a new type with two properties, one being the
// whole OrderItem object.
var orderItemDetail = from o in Orders
from i in Orders.OrderItems
where i.PartId == 100
select new()
{
OrderNumber = o.OrderNumber,
Item = i
}
由于“i”是 OrderItem 类型的对象,因此 Item 被创建为 OrderItem。
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