|
DataLoadOptions
|
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：
|
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：
|
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：
|
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：
|
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：
|
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？
|
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：
|
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]
|
|