Mapping Object Hibernate

Question

I'm trying to insert hibernate in my project but i have a problem with the mapping. My db contains: - id ->int - question -> varchar - result -> varchar - id_left -> int - id_right -> int

here, the code of my bean

@Entity
@Table (name = "node")
public class Nodes
{
    @Id
    @GeneratedValue (strategy = GenerationType.IDENTITY)
    @Column(name="id")
    private int id;

    @Column (name = "question")
    private String question;

    @Column ( name = "result")
    private String result;

    @OneToOne(fetch= FetchType.LAZY)
    private Nodes left;

    @OneToOne(fetch= FetchType.LAZY)
    private Nodes right;   

    public Nodes(int id, String question, String result, Nodes left, Nodes right)
    {
           this.id=id;
           this.question=question;
           this.result=result;
           this.left=left;
           this.right=right;
    }

    public Nodes()
    {

    }

        public Nodes getLeftNodes()
    {
        return left;
    }

    public void setLeftNodes(Nodes left)
    {
        this.left=left;
    }

    public Nodes getRightNodes()
    {
        return right;
    }

    public void setRifhtNodes(Nodes right)
    {
        this.right=right;
    }

and here my Nodes.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class name="beans.Nodes" table="node">
        <id name="id" type="int" access="field">
            <column name="id" />
            <generator class="assigned" />
        </id>
        <property name="question" type="java.lang.String">
            <column name="question" />
        </property>
        <property name="result" type="java.lang.String">
            <column name="result" />
        </property>
        <one-to-one name="left" class="beans.Nodes" access="field"></one-to-one>
        <one-to-one name="right" class="beans.Nodes" access="field"></one-to-one>
    </class>
</hibernate-mapping>

and my display

public Nodes select (HttpServletRequest request) throws Dao_exception
    {
        List <Nodes> nodeList = new ArrayList<Nodes>();
        nodeList = em.createQuery("SELECT n FROM Nodes n").getResultList();

        return nodeList.get(0);
    }

In fact, it would make 2 connections one-to-one I showed you earlier with my 2 items left / right of my bean.

Why? Because I want to build a binary tree of my game This is a result of a question, if I answer "yes", I left, otherwise right.

My problem is (in my opinion) the mapping of my class. Indeed, when I run the program, eclipse makes me the big eyes and leaves me this exception

Call: SELECT id, question, result, LEFT_id, RIGHT_id FROM node
Query: ReadAllQuery(referenceClass=Nodes sql="SELECT id, question, result, LEFT_id, RIGHT_id FROM node")

I understand that the application is malfunctioning because in my database I have no left / right_id. But how can I tell the program that id_left / right corresponds to 2 items left / right and therefore become my request.

Call: SELECT id, question, result, id_left, id_right FROM node

Answer 1

Try using < many-to-one > mapping instead of < one-to-one >. The foreign key column can be specified in < many-to-one >. e.g.,

< many-to-one column="id_left" name="left" class="beans.Nodes">< /many-to-one>