一个查询中很少有 GroupJoin

Question

我正在尝试编写具有很少一致的 GroupJoin 的 ObjectQuery，这意味着应该有一个主表选择 + 很少的附加 LEFT JOIN。我使用 SelectMany 方法按如下方式执行此操作，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 方法会导致此行为。它生成此查询：

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))

我删除了它并收到以下错误：

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 }
                );

方法的类型参数 'System.Linq.Enumerable.SelectMany(System.Collections.Generic.IEnumerable, 系统.Func>, 系统.Func)' 无法从用法推断。尝试 指定类型参数 明确地。

请有人向我解释一下这种行为，并建议如何解决它（如果可能）。

提前致谢。

Answer 1

您正在尝试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：你不必每次都投影到匿名类型：

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

可能是

(Routes, Locales) => Routes

Answer 2

好吧，继续折磨 LINQ 可能会很有用，尽管现在看起来毫无用处，而且 LINQ 可能只适合包含一个表的短查询。在我看来，它是为数组和 XML 创建的。

对于真实 SQL 查询，最好使用在 EF 实体内初始化的直接 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：

• http://www.simple-talk.com/dotnet/.net-framework/entity-framework-the- cribsheet/
• http://www.scip.be/index.php ?Page=ArticlesNET12#生成