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
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var products = from p in ctx.Products select p; foreach (var p in products) { if (p.UnitPrice > 10) ShowDetail(p.Order_Details); }
private void ShowDetail(EntitySet<Order_Detail> orderdetails) {} |
由于ShowDetail方法并没有使用到订单详细信息，所以这个操作只会执行下面的SQL：
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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] FROM [dbo].[Products] AS [t0] |
现在修改一下ShowDetail方法：
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private void ShowDetail(EntitySet<Order_Detail> orderdetails) { foreach (var o in orderdetails) { Response.Write(o.Quantity + "<br>"); } } |
你会发现Linq to sql对每个价格大于10的产品都根据产品号进行了一次查询：
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SELECT [t0].[OrderID], [t0].[ProductID], [t0].[UnitPrice], [t0].[Quantity], [t0].[Discount] FROM [dbo].[Order Details] AS [t0] WHERE [t0].[ProductID] = @p0 -- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [1] |
这样的语句查询了N次。这样的查询不是很合理，我们可以通过设置DataContext的DataLoadOption，来指示 DataContext再加载产品信息的同时把对应的产品订单信息一起加载：
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DataLoadOptions options = new DataLoadOptions(); options.LoadWith<Product>(p => p.Order_Details); ctx.LoadOptions = options; var products = from p in ctx.Products select p; 。。。。。。。。 |
再执行先前的查询会发现Linq to sql进行了左连接：
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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], [t1].[OrderID], [t1].[ProductID] AS [ProductID2], [t1].[UnitPrice] AS [UnitPrice2], [t1].[Quantity], [t1].[Discount], ( SELECT COUNT(*) FROM [dbo].[Order Details] AS [t2] WHERE [t2].[ProductID] = [t0].[ProductID] ) AS [count] FROM [dbo].[Products] AS [t0] LEFT OUTER JOIN [dbo].[Order Details] AS [t1] ON [t1].[ProductID] = [t0].[ProductID] ORDER BY [t0].[ProductID], [t1].[OrderID] |
那么，我们怎么限制订单详细表的加载条件那？
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DataLoadOptions options = new DataLoadOptions(); options.LoadWith<Product>(p => p.Order_Details); options.AssociateWith<Product>(p => p.Order_Details.Where(od => od.Quantity > 80)); ctx.LoadOptions = options; var products = from p in ctx.Products select p; |
这样，就只会有数量大于80的订单详细信息会和产品一起加载：
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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], [t1].[OrderID], [t1].[ProductID] AS [ProductID2], [t1].[UnitPrice] AS [UnitPrice2], [t1].[Quantity], [t1].[Discount], ( SELECT COUNT(*) FROM [dbo].[Order Details] AS [t2] WHERE ([t2].[Quantity] > @p0) AND ([t2].[ProductID] = [t0].[ProductID]) ) AS [count] FROM [dbo].[Products] AS [t0] LEFT OUTER JOIN [dbo].[Order Details] AS [t1] ON ([t1].[Quantity] > @p0) AND ([t1].[ProductID] = [t0].[ProductID]) ORDER BY [t0].[ProductID], [t1].[OrderID] -- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [80] |