Question

我在Visual Studio 2010 Beta 2（.NET framework 4.0 Beta 2）中使用Entity Framework。我已经从我的数据库中创建了一个实体框架.edmx模型，并且我有一些多对多的关系。

我的数据库架构的一个简单示例是

角色（身份证，姓名，有效）
会员（ID，DateOfBirth，DateCreated）
RoleMembership（RoleID，MemberID）

我现在正在编写自定义角色提供程序（继承System.Configuration.Provider.RoleProvider）并且已经编写了IsUserInRole（username，roleName）的实现。

我写的LINQ-to-Entity查询，当SQL-Profiled时，所有生成CROSS JOIN语句的时候我想要的是INNER JOIN。

        Dim query = From m In dc.Members
                    From r In dc.Roles
                    Where m.ID = 100 And r.Name = "Member"
                    Select m

我的问题几乎在这里描述： Entity framework and many to many queries unusable?

我确信那里提供的解决方案效果很好，但是当我在uni学习Java时，我几乎可以理解C＃，我无法理解提供的Lambda语法，我需要在VB中获得类似的示例。我在网上浏览了半天的最佳时间，但我并没有接近我的答案。

所以请有人建议如何在VB中构建一个LINQ语句，它在SQL中可以做到这一点：

SELECT rm.RoleID
FROM RoleMembership rm 
  INNER JOIN Roles r ON r.ID = rm.RoleID
  INNER JOIN Members m ON m.ID = rm.MemberID
WHERE r.Name = 'Member' AND m.ID = 101

我会使用此查询来查看成员101是否在角色3中。（我很感激我可能不需要在SQL中加入Members表，但我想在LINQ中我需要引入Member对象吗？）

更新

使用多种方法让我更接近：

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    Dim count As Integer

    Using dc As New CBLModel.CBLEntities

        Dim persons = dc.Members.Where(AddressOf myTest)

        count = persons.Count

    End Using

    System.Diagnostics.Debugger.Break()
End Sub

Function myTest(ByVal m As Member) As Boolean
    Return m.ID = "100" AndAlso m.Roles.Select(AddressOf myRoleTest).Count > 0
End Function

Function myRoleTest(ByVal r As Role) As Boolean
    Return r.Name = "Member"
End Function

SQL事件探查器显示：

SQL：BatchStarting

SELECT 
[Extent1].[ID] AS [ID], 
... (all columns from Members snipped for brevity) ...
FROM [dbo].[Members] AS [Extent1]

RPC：已完成

exec sp_executesql N'SELECT 
[Extent2].[ID] AS [ID], 
[Extent2].[Name] AS [Name], 
[Extent2].[Active] AS [Active]
FROM  [dbo].[RoleMembership] AS [Extent1]
INNER JOIN [dbo].[Roles] AS [Extent2] ON [Extent1].[RoleID] = [Extent2].[ID]
WHERE [Extent1].[MemberID] = @EntityKeyValue1',N'@EntityKeyValue1 int',@EntityKeyValue1=100

SQL：BatchCompleted

SELECT 
[Extent1].[ID] AS [ID], 
... (all columns from Members snipped for brevity) ...
FROM [dbo].[Members] AS [Extent1]

我不确定为什么它使用sp_execsql作为内部连接语句以及为什么它仍然运行select以选择所有成员。

感谢。

更新2

我通过将上面的“多个方法”转换为lambda表达式然后全部放入一个查询来编写它，如下所示：

    Dim allIDs As String = String.Empty

    Using dc As New CBLModel.CBLEntities

        For Each retM In dc.Members.Where(Function(m As Member) m.ID = 100 AndAlso m.Roles.Select(Function(r As Role) r.Name = "Doctor").Count > 0)
            allIDs &= retM.ID.ToString & ";"
        Next

    End Using

但它似乎不起作用：“Doctor”不是一个存在的角色，我只是把它放在那里用于测试目的，但“allIDs”仍然设置为“100;”

SQL Profiler中的SQL这次看起来像这样：

SELECT 
[Project1].*
FROM ( SELECT 
    [Extent1].*, 
    (SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[RoleMembership] AS [Extent2]
        WHERE [Extent1].[ID] = [Extent2].[MemberID]) AS [C1]
    FROM [dbo].[Members] AS [Extent1]
)  AS [Project1]
WHERE (100 = [Project1].[ID]) AND ([Project1].[C1] > 0)

为简洁起见，我将Members表中所有列的列表转换为*

正如您所看到的，它只是忽略了“角色”查询。

Answer 1

如果您未在架构中使用关联（映射或外键），则可以使用此sintax：

Dim query = From rm As RoleMembership _
            In RoleMemberships _
            Join m As Member In Members On m.ID Equals rm.MemberID _
            Join r As Role In Roles On r.ID Equals rm.RoleID _
            Where r.Name = "Member" _
                And m.ID = 100 _
            Select rm

如果您以这种方式使用关联：

Dim query2 = From r As Role _
             In Roles _
             Where r.Name = "Member" _
                 And r.Members.Any(Function(m As Member) m.ID = 100) _
             Select r

Answer 2

这是我使用Lambda表达式解决它的方法：

    Public Overrides Function IsUserInRole(ByVal username As String, ByVal roleName As String) As Boolean
        Dim retVal As Boolean

        Using db As New CBLEntities
            Dim theRole = db.Roles.Where(Function(x) x.Name = roleName)
            retVal = theRole.Any(Function(r As Role) r.Members.Where(Function(m As Member) m.ID = username).Any())
        End Using

        Return retVal
    End Function

实体框架使用VB.Net Lambda进行多对多

2 个答案: