为什么EF 4不为FK关系生成与具有唯一索引的列的关联?

我遇到过这样一个场景:EntityFramework4.0不会生成与由具有 索引,我想知道为什么。

基本设置如下:假设我在SQL Server 2008 R2中有两个表和一个外键关系:

CREATE TABLE [dbo].[User](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [GroupId] [int] NULL,
 CONSTRAINT [PK_User] PRIMARY KEY CLUSTERED 
(
    [Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF,
ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

CREATE TABLE [dbo].[Group](
[Id] [int] IDENTITY(1,1) NOT NULL,
 CONSTRAINT [PK_Group] PRIMARY KEY CLUSTERED 
(
    [Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF,
    ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

ALTER TABLE [dbo].[User]  WITH CHECK ADD  CONSTRAINT [FK_User_Group] 
    FOREIGN KEY([GroupId])
REFERENCES [dbo].[Group] ([Id])

此外,假设存在以下索引:

CREATE NONCLUSTERED INDEX [IX_Group] ON [dbo].[Group] 
(
[Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]

如果我在VisualStudio2010中告诉设计师生成ADO.NET 实体数据模型我得到一个有两个类的模型, User Group , 用户 组

现在,让我们假设索引是这样的:

CREATE UNIQUE NONCLUSTERED INDEX [IX_Group] ON [dbo].[Group] 
(
    [Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]

也就是说 我所做的就是使索引成为唯一的索引。这样做之后,当我告诉visualstudio的设计者生成一个实体模型时,用户和组之间的关联并没有显示出来 用户 没有导航属性。检查生成的EDMX文件会发现存储模型根本没有关联。

有人能解释这是为什么吗?为什么唯一索引阻止EF对关系建模?

非常感谢。