EntityFramework Code First self-referencing one to one (1:1) relationship mapping - Unable to determine the principal end

Question

I have this class:

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

    public string Subject { get; set; }

    public string Message { get; set; }

    public virtual Message ParentMessage { get; set; }

    public virtual Message ChildMessage { get; set; }

    //...
}

Using EntityFramework Code First Add-Migration gives me the message: Unable to determine the principal end of an association between the types...

I can't use the [Required] attribute, because the first message in this thread will have no parent, the last message in the thread will have no child... how do I map this?

I tried:

        modelBuilder.Entity<Message>()
            .HasOptional(x => x.ParentMessage);

        modelBuilder.Entity<Message>()
            .HasOptional(x => x.ChildMessage);

but that did not work.

Answer 1

I found something that looks like it could be it, if someone could verify that this is correct:

        modelBuilder.Entity<SecureMessage>()
            .HasOptional(x => x.ParentMessage)
            .WithOptionalDependent(x => x.ChildMessage);

So after some serious testing, this does indeed seem to be the solution.