但是，当我试图从视频2删除现有的同事（B和C），
和尝试添加同事A，那么关系不会被更新。

块引用>

如果不使用通用的存储库中的正确程序是

 使用（VAR会话=新的DatabaseSession（））
 {
视频2 = session.Context。设置<视频方式>（）包含（v =&GT; v.Coworkers）
。单（v =&GT; v.Id == video2Id）; 

 coworkerA =新同事{ID = coworkerAId}; 
 session.Context.Set&LT;同事方式>（）将（coworkerA）; 

 video2.Coworkers.Clear（）; 
 video2.Coworkers.Add（coworkerA）

 session.Context.SaveChanges（）; 
} 
 

关键部分是，你必须加载或附加实体在其原始状态，改变实体，即删除和添加的孩子，然后保存更改。 EF的变化检测将创造必要的插入和删除的链接表项的语句。简单的过程状态设置为修改您想在您的泛型更新方法仅适用于更新标性质 - 如更改视频节目 - 但不会更新实体之间的关系的工作。

I am using Entity Framework 4.3 Code First, and I have problem with updating many-to-many relationships.

I defined the following classes:

public abstract class Entity
{
    [Column(Order = 0)]
    [Key]
    public int Id { get; set; }

    [Timestamp]
    [Column(Order = 1)]
    public byte[] Version { get; set; }
}


public class Video : Entity
{
    public string Title { get; set; }
    public string Description { get; set; }
    public TimeSpan Length { get; set; }

    public virtual ICollection<Coworker> Coworkers { get; set; }
}


public class Coworker : Entity
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public virtual ICollection<Video> Videos { get; set; }
}

When the database is created, the schema look right: There is a Videos, Coworkers and VideoCoworkers table too, without

I use repository pattern in an N-Tier application to access database, my Insert and Update method looks like this:

public T Insert(T entity)
{
    //Creates database context. When it disposes, it calls context.SaveChanges()
    using (var session = new DatabaseSession())
    {
        session.Context.Set<T>().Add(entity);
    }
}

public T Update(T entity)
{
    //Creates database context. When it disposes, it calls context.SaveChanges()
    using (var session = new DatabaseSession())
    {
        entity = session.Context.Set<T>().Attach(entity);
        session.Context.Entry(entity).State = EntityState.Modified;
    }
    return entity;
}

When I update an entity, I create the entity object from a DTO, that's why use DbSet.Attach instead of selecting it and updating the properties one-by-one.

When I initialize the database, I add some test data:

1. Create 3 Coworkers, where I set first and last name. (A, B, C)
2. Create 3 Videos, where I set title, description and length, and also set some coworkers. First video has A,B, second has B,C and third has A,C.

When I list the Videos from code, I can see that Video.Coworkers collection is filled with good values, and when I query the link table (VideoCoworkers) in SQL Server Management Studio, it also looks good.

My problem is when I update for example the title of the Video, it works. But when I try to delete from Video2 the existing coworkers (B and C), and try to add coworker A, then the relationship is not updated. It also does not work when I only try to add new coworker, or only try to delete one. I create the entity which is used as the parameter of the Update() method by creating a new Video entity with a new collection of Coworkers (which are selected from the database with Find() method by Id).

What is the correct way to update many-to-many relationships?

But when I try to delete from Video2 the existing coworkers (B and C), and try to add coworker A, then the relationship is not updated.

Without using a generic repository the correct procedure would be:

using (var session = new DatabaseSession())
{
    video2 = session.Context.Set<Video>().Include(v => v.Coworkers)
        .Single(v => v.Id == video2Id);

    coworkerA = new Coworker { Id = coworkerAId };
    session.Context.Set<Coworker>().Attach(coworkerA);

    video2.Coworkers.Clear();
    video2.Coworkers.Add(coworkerA)

    session.Context.SaveChanges();
}

The essential part is that you must load or attach the entity in its original state, change the entity, i.e. remove and add children, and then save the changes. EF's change detection will create the necessary INSERT and DELETE statements for the link table entries. The simple procedure to set the state to Modified you are trying in your generic Update method is suited only for updating scalar properties - like changing the video title - but won't work for updating relationships between entities.