实体框架不会将数据库上下文中仍然存在的对象与从不同类引用的同一对象相关联

Question

I'm trying to add a message to my database
Message.cs:

public class Message
{
    public int Id { get; set; }

    public int SenderId { get; set; }

    public virtual User Sender { get; set; }

    public virtual Collection<Recipient> Recipients { get; set; }

    public MessageTrigger Trigger { get; set; }

    public string Body { get; set; }

    public DateTime CreatedTs { get; set; }
}

I have also a table named Recipients
Recipient.cs:

public class Recipient
{
    public int Id { get; set; }

    public int MessageId { get; set; }

    public int UserId { get; set; }

    public virtual User User { get; set; }

    public DateTime? DeliviredOn { get; set; }
}

As you see both tables have their foreign key set to the Id of an User
User.cs:

public class User
{
    public int Id { get; set; }

    public string FirstName { get; set; }

    public string LastName { get; set; }
}

Now while adding a new Message to the database

 public void Add(Message message)
 {          
     using (var ctx = new ShowcaseContext())
     {
         foreach (var reci in message.Recipients)
         {
             ctx.Users.Attach(reci.User);
         }

         ctx.Messages.Add(message);
         ctx.SaveChanges();
     }
 }

I have to attach all the user to the context, because if not EF is adding the User again to the database. This worked all fine while Message doesn't had a Sender . Now after adding the Sender and SenderId properties to the Message class, EF adds the user again to the user table. So I tried something like that:

 public void Add(Message message)
 {          
     using (var ctx = new ShowcaseContext())
     {
         foreach (var reci in message.Recipients)
         {
             ctx.Users.Attach(reci.User);
         }

         ctx.Users.Attach(message.Sender);  // Here the error is thrown
         ctx.Messages.Add(message);
         ctx.SaveChanges();
     }
 }

But then this Error occures:

Error attaching an entity of type Entities.User, because another entity of the same type already has an identical primary key value

So the user of sender is already in the database context, but the context doesn't seem to associate this user with the user of the sender .
And because of that the userobject of sender is added to the database as an duplicate.

Is there someone out there who knows a solution for this problem?
I just want to add a message to the database without EF adding an already existing user again to the user table.

Answer 1

I have solved the problem with this:

public void Add(Message message)
{          
    using (var ctx = new ShowcaseContext())
    {
        foreach (var reci in message.Recipients)
        {
            ctx.Users.Attach(reci.User);
        }

        ctx.Messages.Add(message);
        ctx.Entry(message.Sender).State = EntityState.Detached;
        ctx.SaveChanges();
    }
}

After adding the message to the context, I explicitly detach the sender user object from the context.
I guess it is not the best solution, but the only one I can think of which works.