Java Tree with Ebean (parent child relation in same table)

I am completely stuck with retrieving a parent child structure from the database. I cannot create a descent Model that will work with Ebean evolutions because the parent relationship is not recognized by Ebean. What I tried:

public class Category extends Model {

    private static final long serialVersionUID = 4660222569406895990L;

    private Long id;
    private String sfname;
    // @ManyToOne -- does not work
    // @JoinColumn(name="id")
    private Category parent;

    public static Finder<Integer, Category> FIND = new Finder<Integer, Category>(Integer.class, Category.class);
}

No matter what I try, the code above is generating the following SQL Evolution script:

create table category (
   id                        bigint,
   name                      varchar(255)
);

And if the above will works, the top level has no parent, so how would or can this work??

I have completely no experience with trees and java, perhaps that why it is so difficult for me :-) Thanks for your help!

** 2013-03-28 EDIT **

As nico pointed out, I forgot to add @Id to the primary key. So the, following model, works:

@Id
private Long id;
private String sfname;
@ManyToOne
private Category parent;

You may have forgotten the @Id annotation under private Long id:

public class Category extends Model {

    private static final long serialVersionUID = 4660222569406895990L;

    @Id
    private Long id;
    ...
}