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我正在尝试使用一些一致的 GroupJoin 编写 ObjectQuery，这意味着应该有一个主表选择 + 一些额外的 LEFT JOIN。我正在使用 SelectMany 方法执行以下操作，因为没有它我无法访问字段 RoleID ：

var routesQuery = entities.Routes.Join(
                    entities.Locales,
                    Routes => Routes.LocaleID,
                    Locales => Locales.LocaleID,
                    (Routes, Locales) => new { Routes = Routes }
                ).GroupJoin(
                    entities.LinkRolesPermissions,
                    Routes => Routes.Routes.RouteID,
                    LinkRolesPermissions => LinkRolesPermissions.EntityID,
                    (Routes, LinkRolesPermissions) => new 
                    { 
                        LinkRolesPermissions = LinkRolesPermissions,
                        RoleID = LinkRolesPermissions.SelectMany(
                            LRS => LRS.RoleID,
                            (LRS, RoleID) => new { RoleID = LRS.RoleID }
                        )
                    }
                )
                .SelectMany(
                    LinkRolesPermissions => LinkRolesPermissions.RoleID, 
                    (LinkRolesPermissions, RoleID) => new { RoleID = RoleID }
                 ).GroupJoin(
                    entities.aspnet_Roles, 
                    LRS => LRS.RoleID, 
                    RLS => RLS.RoleId, 
                    (LRS, RLS) => new { LRS = LRS }
                );

一切正常，但它以某种方式作为 INNER JOIN 工作，我意识到 SelectMany 方法会导致这种行为。它生成这个查询：

SELECT  1 AS [C1],  1 AS [C2],  [Extent3].[RoleID] AS [RoleID] FROM   [dbo].[Routes] AS [Extent1] 
INNER JOIN [dbo].[Locales] AS [Extent2] ON ([Extent1].[LocaleID] = [Extent2].[LocaleID]) OR (([Extent1].[LocaleID] IS NULL) AND ([Extent2].[LocaleID] IS NULL)) 
INNER JOIN [dbo].[LinkRolesPermissions] AS [Extent3] ON ([Extent1].[RouteID] = [Extent3].[EntityID]) OR (([Extent1].[RouteID] IS NULL) AND ([Extent3].[EntityID] IS NULL))

我将其删除并收到以下错误：

var routesQuery = entities.Routes.Join(
                    entities.Locales,
                    Routes => Routes.LocaleID,
                    Locales => Locales.LocaleID,
                    (Routes, Locales) => new { Routes = Routes }
                ).GroupJoin(
                    entities.LinkRolesPermissions,
                    Routes => Routes.Routes.RouteID,
                    LinkRolesPermissions => LinkRolesPermissions.EntityID,
                    (Routes, LinkRolesPermissions) => new 
                    { 
                        LinkRolesPermissions = LinkRolesPermissions,
                        RoleID = LinkRolesPermissions.SelectMany(
                            LRS => LRS.RoleID,
                            (LRS, RoleID) => new { RoleID = LRS.RoleID }
                        )
                    }
                )
                .GroupJoin(
                    entities.aspnet_Roles, 
                    LRS => LRS.RoleID, 
                    RLS => RLS.RoleId, 
                    (LRS, RLS) => new { LRS = LRS }
                );

无法从用法中推断方法“System.Linq.Enumerable.SelectMany(System.Collections.Generic.IEnumerable, System.Func>, System.Func)”的类型参数。尝试明确指定类型参数。

请有人向我解释这种行为，并建议如果可能的话如何解决它。

提前致谢。

4

2 回答 2

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您正试图急切地加载相关对象。使用 Include 方法来做到这一点。

http://msdn.microsoft.com/en-us/library/bb896272.aspx

IQueryable<Route> query = 
  from route in entities.Routes.Include("LinkRolesPermissions.aspnet_Roles")
  where route.Locales.Any()
  select route;

为此，您必须设置导航属性。

http://msdn.microsoft.com/en-us/library/bb738520.aspx

PS：您不必每次都投射到匿名类型：

(Routes, Locales) => new { Routes = Routes }

可能

(Routes, Locales) => Routes
于 2010-10-27T16:35:36.373 回答
0

好吧，继续折磨 LINQ 可能很有用，尽管现在它似乎没用，而且 LINQ 可能仅适用于包含一个表的短查询。它是为数组和 XML、IMO 创建的。

对于真正的SQL 查询，最好使用在 EF 实体中初始化的直接 SQL 查询，如下所示：

ObjectQuery<DbDataRecord> query = new ObjectQuery<DbDataRecord>
(
"SELECT c.Name,c.City, d.Name FROM ContosoEntities.Customer as c " + 
"LEFT JOIN ContosoEntities.Products as d ON d.Name = c.Name " + 
"WHERE c.Country=@ctry", ent
);
query.Parameters.Add(new ObjectParameter("ctry", "Australia"));

使用此处列出的所有 EF 变体：

于 2011-02-26T12:36:56.727 回答