相信没有办法实现这一点。
您可以指定该课程实体不变，而不是按照以下原则添加：

  ctx.Entry（course） .State = EntityState.Unchanged; 
 

或指示您的上下文，您正在使用现有实体：

  ctx.Courses.Attach（course）; 
 

更多信息：
http://msdn.microsoft.com/en-us/data/jj592676.aspx

编辑

我的解决方案中有一些运行的示例，我验证他们按预期工作。
在所有情况下，我们在ID = 2和Name =Addison Wesley的数据库中都有Publisher记录（与示例无关，但仅适用于较好的度量）。

方法1 - 设置实体状态

  using（var context = new Context（））
 {
 var book = new Book（）; 
 book.Name =服务设计模式; 
 book.Publisher = new Publisher（）{Id = 2}; //只需要ID 
 context.Entry（book.Publisher）.State = EntityState.Unchanged; 
 context.Books.Add（book）; 
 context.SaveChanges（）; 
} 
 

方法2 - 使用附加方法

  using（var context = new Context（））
 {
 var book = new Book（）; 
 book.Name =服务设计模式; 
 book.Publisher = new Publisher（）{Id = 2}; //只需要ID 
 context.Publishers.Attach（book.Publisher）; 
 context.Books.Add（book）; 
 context.SaveChanges（）; 
} 
 

方法3 - 设置外键值 （var context = new Context（））
{
var book = new Book（）;


$ b

  
 book.Name =服务设计模式; 
 book.PublisherId = 2; 
 context.Books.Add（book）; 
 context.SaveChanges（）; 
} 
 

对于最后一种工作方式，我需要添加额外的属性PublisherId，它有根据NavigationPropertyName +'Id约定命名为EF自动提取：

  public int PublisherId {get; set ;} 
 public Publisher Publisher {get; set;} 
 

我在这里使用EF5代码首先，但与POCO非常相似。

Im wanting to use Entity Framework POCO in a disconnected (from context) mode. In my scenario I'm creating a new Parent object and want to attach an existing child object to it and then save it to the db.
The code below undesirably inserts a new Course record when saving a new Student record, when instead I want the existing Course record linked to the new Student record.

How can I do this in Entity Framework where...

• the objects can be disconnected from the context. (i.e. Queried in one context and then saved in a different context)
• I dont need to re-query the child record from the DB just so I can attach it to the parent when I'm saving to db. I really want to avoid doing extra trips to the db when I already have it as an object in memory.

This page shows a database diagram that the code below is based on http://entityframeworktutorial.net/EF4_EnvSetup.aspx#.UPMZ4m-UN9Y

class Program
{
    static void Main(string[] args)
    {

        //get existing course from db as disconnected object
        var course = Program.getCourse();

        //create new student
        var stud = new Student();
        stud.StudentName = "bob";

        //assign existing course to the student
        stud.Courses.Add(course);

        //save student to db
        using (SchoolDBEntities ctx = new SchoolDBEntities())
        {
            ctx.Students.AddObject(stud);
            ctx.SaveChanges();
        }
    }

    static Course getCourse()
    {
        Course returnCourse = null;

        using (var ctx = new SchoolDBEntities())
        {
            ctx.ContextOptions.LazyLoadingEnabled = false;
            returnCourse = (from s in ctx.Courses
                            select s).SingleOrDefault();
        }

        return returnCourse;
    }
}

I believe there are few ways of accomplishing this. You can specify that course entity is unchanged rather than added, along these lines:

ctx.Entry(course).State = EntityState.Unchanged;

Or instruct your context, that you are working with existing entity:

ctx.Courses.Attach(course);

More info here: http://msdn.microsoft.com/en-us/data/jj592676.aspx

EDIT

There are some running samples from my solution, I verified they work as expected. In all cases we have Publisher record in database with ID = 2 and Name = "Addison Wesley" (irrelevant to the example, but just for good measure).

Approach 1 - Setting Entity State

using (var context = new Context())
{
    var book = new Book();
    book.Name = "Service Design Patterns";
    book.Publisher = new Publisher() {Id = 2 }; // Only ID is required
    context.Entry(book.Publisher).State = EntityState.Unchanged;
    context.Books.Add(book);
    context.SaveChanges();
}

Approach 2 - Using Attach method

using (var context = new Context())
{
    var book = new Book();
    book.Name = "Service Design Patterns";                
    book.Publisher = new Publisher() { Id = 2 }; // Only ID is required
    context.Publishers.Attach(book.Publisher);
    context.Books.Add(book);
    context.SaveChanges();
}

Approach 3 - Setting Foreign Key value

using (var context = new Context())
{
     var book = new Book();
     book.Name = "Service Design Patterns";
     book.PublisherId = 2; 
     context.Books.Add(book);
     context.SaveChanges();
}

For this last approach to work I needed to add extra property PublisherId, it has to be named according to NavigationPropertyName + 'Id" convention to be picked up by EF auotmatically:

public int PublisherId { get; set; }
public Publisher Publisher { get; set; }

I am using here EF5 Code First, but it is very similar to POCO.