如何编写linq查询以防止重复连接？

Question

我有一个查询，搜索订单中的所有住宿，按天排序。 当我在服务器上检查执行了什么查询时，我看到同一个键上的同一个表的多个连接

var parcourt = this.DataService.From<OrderItem>()
                               .Where(i => i.OrderId == orderId && i.Product.ProductTypeId == (int)ProductTypes.Accommodation)
                               .OrderBy(i => i.DayNumber)
                               .ThenBy(i => i.OrderItemId)
                               .Select(i => new
                               {
                                   i.OrderItemId,
                                   i.DayNumber,
                                   i.Product.Establishment.Address,
                                   i.Product.Establishment.Coordinates
                               });

如果检查生成的SQL（由ToTraceString显示），则可以在Products and Establishments表中看到两个连接。

SELECT 
[Project1].[OrderItemId] AS [OrderItemId], 
[Project1].[DayNumber] AS [DayNumber], 
[Project1].[Address] AS [Address], 
[Project1].[EstablishmentId] AS [EstablishmentId], 
[Project1].[Latitude] AS [Latitude], 
[Project1].[Longitude] AS [Longitude]
FROM ( SELECT 
    [Extent1].[OrderItemId] AS [OrderItemId], 
    [Extent1].[DayNumber] AS [DayNumber], 
    [Extent4].[Address] AS [Address], 
    [Extent5].[EstablishmentId] AS [EstablishmentId], 
    [Extent5].[Latitude] AS [Latitude], 
    [Extent5].[Longitude] AS [Longitude]
    FROM     [dbo].[OrderItems] AS [Extent1]
    INNER JOIN [dbo].[Products] AS [Extent2] ON [Extent1].[ProductId] = [Extent2].[ProductId]
    LEFT OUTER JOIN [dbo].[Products] AS [Extent3] ON [Extent1].[ProductId] = [Extent3].[ProductId]
    LEFT OUTER JOIN [dbo].[Establishments] AS [Extent4] ON [Extent3].[EstablishmentId] = [Extent4].[EstablishmentId]
    LEFT OUTER JOIN [dbo].[Establishments] AS [Extent5] ON [Extent3].[EstablishmentId] = [Extent5].[EstablishmentId]
    WHERE (1 = [Extent2].[ProductTypeId]) AND ([Extent1].[OrderId] = @p__linq__0)
)  AS [Project1]
ORDER BY [Project1].[DayNumber] ASC, [Project1].[OrderItemId] ASC

如何防止此linq-to-entities在表上加入两次？ 如何重写查询以避免这种情况？

表结构如下（简化）：

这是查询

Answer 1

你能试试这个查询吗？ 我想如果你明确地调用所有连接，它将不会自动创建连接。

var parcourt = (from i in this.DataService.OrderItem
                join p in this.DataService.Product on p.ProductId equals i.ProductId
                join e in this.DataService.Establishments on e.EstablishmentId equals p.EstablishmentId
                where i.OrderId == orderId && p.ProductTypeId == (int)ProductTypes.Accomodation
                orderby i.DayNumber, i.OrderItemId
                select new
                {
                    i.OrderItemId,
                    i.DayNumber,
                    e.Address,
                    e.Coordinates
                });