EF实现主从表自动生成主键保存

Class cl = new Class() {   ClassName = "一年级1班" };

TestDBEntities context = new TestDBEntities();
context.Class.Add(cl);

Student st = new Student() { Class = cl,  Name="wangxiaoming" };
context.Student.Add(st);
context.SaveChanges();

CREATE TABLE [dbo].[Class](
[ClassID] [int] IDENTITY(1,1) NOT NULL,
[ClassName] [nvarchar](50) NOT NULL,
CONSTRAINT [PK_Class] PRIMARY KEY CLUSTERED
(
[ClassID] 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].[Student](
[StudentID] [int] IDENTITY(1,1) NOT NULL,
[Name] [nvarchar](50) NOT NULL,
[ClassID] [int] NOT NULL,
CONSTRAINT [PK_Student] PRIMARY KEY CLUSTERED
(
[StudentID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]