android - Android 如何在 ORMLite 中建立自我关系？

Question

我有以下对象是自关系的，如下：

@DatabaseTable(tableName = "categoria")
public class Categoria implements Serializable {

    @DatabaseField(generatedId = true)
    public int id;

    @DatabaseField(canBeNull = true, foreign = true)
    public Categoria pai;

    @DatabaseField(canBeNull = false, width = 50, unique = true)
    public String descricao;

    public Categoria() { }
}

当我这样做时：

Categoria cat = new Categoria();
cat.pai = null;
cat.descricao = "Comidas";

categoryDao.create(cat); //should be id = 1.

Categoria sub_cat = new Categoria();
sub_cat.pai = cat;
sub_cat.descricao = "Bebidas";

categoryDao.create(sub_cat); // should be id = 2.

因此，执行此操作时，我的对象类别为空：

Categoria sub_cat = categoryDao.queryForId(2);

sub_cat.descricao // is ok, return "Bebidas".

但

sub_cat.pai // is null when supposed to be sub_cat.pai.id = 1.

我做错了什么？