Bidirectional one-to-one relationships in Hibernate

Question

There are two tables: Airplane and Engine

The Engine table is made up as follows [Engine_ID, Engine_Name,Airplane_Owner_ID].
The Airplane table is made up as follows [Airplane_ID, Left_Engine, Right_Engine]

Left_Engine and Right_Engine are foreign keys from the Engine table, moreover Airplane_Owner_ID is the foreign key from the Airplane table. Therefore there are three one-to-one relationships defined between the Airplane and Engine tables.

I know how to specify single one-to-one relationships between two tables but how can I specify multiple relationships between two tables? Is it the same process?

How can these relationships be specified in Hibernate?

M-Razavi · Accepted Answer

As @APC said, Cyclic dependencies between Engine and Airplane is a bad design. But with the solution described below, Engine.Airplane_Owner_ID could be implemented only as logical back-link which not exist in Database table.

Airplane.hbm.xml

Engine.hbm.xml

Airplane.java

public class Airplane {
    private int id;
    private String name;

    private Engine rightEngine;
    private Engine leftEngine;

    public Airplane(String name) {
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Engine getRightEngine() {
        return rightEngine;
    }

    public void setRightEngine(Engine rightEngine) {
        this.rightEngine = rightEngine;
    }

    public Engine getLeftEngine() {
        return leftEngine;
    }

    public void setLeftEngine(Engine leftEngine) {
        this.leftEngine = leftEngine;
    }

    @Override
    public String toString() {
        return "Airplane{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", rightEngline=" + (rightEngine == null ? null : rightEngine.getName()) +
                ", leftEngine=" + (leftEngine == null ? null : leftEngine.getName()) +
                '}';
    }
}

Engine.java

public class Engine {

    private int id;
    private String name;
    private byte position;//0=left, 1=right
    private Airplane ownerAirplane;

    /**
     * @param name
     * @param position 0=left, 1=right
     */
    public Engine(String name, byte position) {
        this.name = name;
        this.position = position;
    }

    public Airplane getOwnerAirplane() {
        return ownerAirplane;
    }

    public void setOwnerAirplane(Airplane ownerAirplane) {
        this.ownerAirplane = ownerAirplane;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }


    public void setName(String name) {
        this.name = name;
    }

    /**
     * @return 0=left, 1=right
     */
    public byte getPosition() {
        return position;
    }

    /**
     * @param position 0=left, 1=right
     */
    public void setPosition(byte position) {
        this.position = position;
    }


    @Override
    public String toString() {
        return "Engine{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", position=" + position +
                ", ownerAirplane=" + (ownerAirplane == null ? null : ownerAirplane.getName()) +
                '}';
    }
}

Main.java

 public static void main(final String[] args) throws Exception {

        Session session = ourSessionFactory.openSession();
        Transaction transaction = null;
        try {
            transaction = session.beginTransaction();
            Engine engineRight1 = new Engine("engineRight1", (byte) 1);
            Engine engineLeft1 = new Engine("engineLeft1", (byte) 0);
            Airplane airplane1 = new Airplane("Airplane1");

            Engine engineRight2 = new Engine("engineRight2", (byte) 1);
            Engine engineLeft2 = new Engine("engineLeft2", (byte) 0);
            Airplane airplane2 = new Airplane("Airplane2");

            Engine engineRight3 = new Engine("engineRight3", (byte) 1);
            Engine engineLeft3 = new Engine("engineLeft3", (byte) 0);
            Airplane airplane3 = new Airplane("Airplane3");

            engineLeft1.setOwnerAirplane(airplane1);
            engineRight1.setOwnerAirplane(airplane1);
            airplane1.setLeftEngine(engineLeft1);
            airplane1.setRightEngine(engineRight1);

            engineRight2.setOwnerAirplane(airplane2);
            airplane2.setRightEngine(engineRight2);
//            airplane2.setLeftEngine(engineLeft1);

            engineRight3.setOwnerAirplane(airplane3);
            airplane3.setLeftEngine(engineLeft3);

            session.save(airplane1);
            session.save(airplane2);
            session.save(airplane3);

            session.save(engineLeft1);
            session.save(engineLeft2);
            session.save(engineLeft3);
            session.save(engineRight1);
            session.save(engineRight2);
            session.save(engineRight3);

            transaction.commit();
        } catch (HibernateException e) {
            transaction.rollback();
            e.printStackTrace();
        }

Bidirectional one-to-one relationships in Hibernate

Answers (2)

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