一步一步学Linq to sql（六）：探究特性
[2] 一步一步学Linq to sql（六）：探究特性
[3] 一步一步学Linq to sql（六）：探究特性
[4] 一步一步学Linq to sql（六）：探究特性
系列文章导航：
一步一步学Linq to sql（二）：DataContext与实体
DataLoadOptions限制
Linq to sql对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：
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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）：
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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DataLoadOptions没有限制：
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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）：
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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] |