在EF4中, 提供了FK Properties 和 FK Associations的支持，对于association没有了于table mapping的概念,只需要映射到2个实体的属性上.没有FK Properties 的association被称作为Independent Association 。因此最近一周断断续续看了几篇关于FK associations的文章.我不想过多的转述那几篇blog中提到的为何在EF4中做出这样的变化.在EF Design team blog上可以看到许多国外朋友对于FK association的利用与弊的讨论.正因为如此,EF Design team

才采取了一种折中的做法，同时支持FK Properties 和FK association.

下面是我对FK Properties 和FK association所作的练习：

表Department,Course,外键关系

department表字段

在EF4.0中

生成的EDM：

EF4.0还不支持Hierarchyid

双击Assocaition 可以看到下面的窗口，但在vs2008 sp1里面是看不到的，不过在vs2010中选中或不选中include foreign key column in the model

还是有区别的：

因为上图的不同所以，edmx的xml文件也不一样:主要是在csdl中的xml不一样；

选中 include foreign key;

下面的内容是关于如何在程序中使用FKAssociation,IndependentAssociation

static void InsertDepartmentFKAssociation()
       {
           using (FKAssociation.FKAssociationEntities2 context = new FKAssociation.FKAssociationEntities2())
           {
               FKAssociation.Deparment d = new Deparment {
                   DepartmentID=Guid.NewGuid(),
                   Budget="budget",
                   Name="ist",
                   StartDate=DateTime.Now,
                   Administrator="administrator" };

               FKAssociation.Course c = new Course { 
                   CourseID=Guid.NewGuid(),
                   DepartmentID=d.DepartmentID,
                   Status="status",
                   Title="title" };
               context.AddToDeparments(d);
               context.AddToCourses(c);
               context.SaveChanges();

           }
       }

       static void InsertDepartmentIndependentAssociation()
       {
           using (FKAssociation.FKAssociationEntities2 context = new FKAssociation.FKAssociationEntities2())
           {
               FKAssociation.Deparment d = new Deparment
               {
                   DepartmentID = Guid.NewGuid(),
                   Budget = "budget",
                   Name = "ist",
                   StartDate = DateTime.Now,
                   Administrator = "administrator"
               };

               FKAssociation.Course c = new Course
               {
                   CourseID = Guid.NewGuid(),
                   DepartmentID = d.DepartmentID,
                   Status = "status",
                   Title = "title",
                   Deparment=d,
               };

               d.Courses.Add(c);                          
               context.SaveChanges();

           }
       }

在使用的时候 其实有很多细节可以体现出2者的不同。在一个edm中的Association是可以更据实际情况来决定是否使用IndependentAssociation 还是FKAssociation。

参考文献

http://blogs.msdn.com/efdesign/archive/2008/10/27/foreign-keys-in-the-conceptual-and-object-models.aspx

http://blogs.msdn.com/efdesign/archive/2009/03/16/foreign-keys-in-the-entity-framework.aspx

http://blogs.msdn.com/b/adonet/archive/2009/11/06/foreign-key-relationships-in-the-entity-framework.aspx

http://blogs.msdn.com/b/msdnforum/archive/2010/05/07/foreign-key-association-in-entity-framework-4.aspx

entity framework 4.0 recipes