Linq DataLoadOptions限制
Linq to sql对Linq DataLoadOptions的使用是有限制的，它只支持1个1对多的关系。一个顾客可能有多个订单，一个订单可能有多个详细订单：
- DataLoadOptions options = new DataLoadOptions();
- options.LoadWith<Customer>(c => c.Orders);
- options.LoadWith<Order>(o => o.Order_Details);
- ctx.LoadOptions = options;
- IEnumerable<Customer> customers = ctx.Customers.ToList<Customer>();
这样的语句执行后会导致下面的SQL执行N次（参数不同）：
- SELECT [t0].[OrderID], [t0].[CustomerID], [t0].[EmployeeID], [t0].[OrderDate],
[t0].[RequiredDate], [t0].[ShippedDate], [t0].[ShipVia], [t0].[Freight], [t0].
[ShipName], [t0].[ShipAddress], [t0].[ShipCity], [t0].[ShipRegion], [t0].
[ShipPostalCode], [t0].[ShipCountry], [t1].[OrderID] AS [OrderID2], [t1].
[ProductID], [t1].[UnitPrice], [t1].[Quantity], [t1].[Discount], (- SELECT COUNT(*)
- FROM [dbo].[Order Details] AS [t2]
- WHERE [t2].[OrderID] = [t0].[OrderID]
- ) AS [count]
- FROM [dbo].[Orders] AS [t0]
- LEFT OUTER JOIN [dbo].[Order Details] AS [t1] ON [t1].[OrderID] = [t0].[OrderID]
- WHERE [t0].[CustomerID] = @x1
- ORDER BY [t0].[OrderID], [t1].[ProductID]
- -- @x1: Input StringFixedLength (Size = 5; Prec = 0; Scale = 0) [ALFKI]
而对于多对1的关系，Linq to sql对于Linq DataLoadOptions没有限制：
- DataLoadOptions options = new DataLoadOptions();
- options.LoadWith<Product>(c => c.Category);
- options.LoadWith<Product>(c => c.Order_Details);
- options.LoadWith<Order_Detail>(o => o.Order);
- ctx.LoadOptions = options;
- IEnumerable<Product> products = ctx.Products.ToList<Product>();
由于多个产品对应1个分类，多个详细订单对应1个订单，只有产品和详细订单才是多对1的关系，所以也只会有1次SQL（不过这样的操作还是少执行为妙，消耗太大了）
- SELECT [t0].[ProductID], [t0].[ProductName], [t0].[SupplierID], [t0].
[CategoryID], [t0].[QuantityPerUnit], [t0].[UnitPrice], [t0].
[UnitsInStock], [t0].[UnitsOnOrder], [t0].[ReorderLevel], [t0].
[Discontinued], [t3].[OrderID], [t3].[ProductID] AS [ProductID2], [t3].
[UnitPrice] AS [UnitPrice2], [t3].[Quantity], [t3].[Discount], [t4].
[OrderID] AS [OrderID2], [t4].[CustomerID], [t4].[EmployeeID], [t4].
[OrderDate], [t4].[RequiredDate], [t4].[ShippedDate], [t4].[ShipVia],
[t4].[Freight], [t4].[ShipName], [t4].[ShipAddress], [t4].[ShipCity],
[t4].[ShipRegion], [t4].[ShipPostalCode], [t4].[ShipCountry], (- SELECT COUNT(*)
- FROM [dbo].[Order Details] AS [t5]
- INNER JOIN [dbo].[Orders] AS [t6] ON [t6].[OrderID] = [t5].[OrderID]
- WHERE [t5].[ProductID] = [t0].[ProductID]
- ) AS [count], [t2].[test], [t2].[CategoryID] AS [CategoryID2], [t2].
[CategoryName], [t2].[Description], [t2].[Picture]- FROM [dbo].[Products] AS [t0]
- LEFT OUTER JOIN (
- SELECT 1 AS [test], [t1].[CategoryID], [t1].[CategoryName], [t1].
[Description], [t1].[Picture]- FROM [dbo].[Categories] AS [t1]
- ) AS [t2] ON [t2].[CategoryID] = [t0].[CategoryID]
- LEFT OUTER JOIN ([dbo].[Order Details] AS [t3]
- INNER JOIN [dbo].[Orders] AS [t4] ON [t4].[OrderID] = [t3].
[OrderID]) ON [t3].[ProductID] = [t0].[ProductID]- ORDER BY [t0].[ProductID], [t2].[CategoryID], [t3].[OrderID]
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