package com.jpa.test;

import java.util.*;
import javax.persistence.*;

@Entity
public class Parent {

    @Id
    private int parentId;
    private String parentDescription;

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "parent")
    private List<Child> children;

    public Parent() {
        children = new ArrayList<Child>();
    }

    public Parent(int parentId, String parentDescription) {
        this.parentId = parentId;
        this.parentDescription = parentDescription;
        children = new ArrayList<Child>();
    }

    public int getParentId() {
        return parentId;
    }

    public void setParentId(int parentId) {
        this.parentId = parentId;
    }

    public String getParentDescription() {
        return parentDescription;
    }

    public void setParentDescription(String parentDescription) {
        this.parentDescription = parentDescription;
    }

    public List<Child> getChildren() {
        return children;
    }

    public void setChildren(List<Child> children) {
        this.children = children;
    }

    @Override
    public String toString() {
        StringBuffer toStringValue = new StringBuffer().append("Parent[");
        toStringValue.append(parentId).append('.');
        toStringValue.append(parentDescription).append(']');
        return toStringValue.toString();
    }
}

package com.jpa.test;

import javax.persistence.*;

@Entity
@IdClass(ChildPK.class)
public class Child {

    @Id
    private int parentId;
    @Id
    private int childId;
    private String childDescription;
    @ManyToOne
    @JoinColumn(name = "parentId", nullable = false, updatable = false, insertable = false)
    private Parent parent;

    public Child() {
    }

    public Child(Parent parent, int childId, String childDescription) {
        parentId = parent.getParentId();
        this.childId = childId;
        this.childDescription = childDescription;
        this.parent = parent;
    }

    public int getParentId() {
        return parentId;
    }

    public void setParentId(int parentId) {
        this.parentId = parentId;
    }

    public int getChildId() {
        return childId;
    }

    public void setChildId(int childId) {
        this.childId = childId;
    }

    public String getChildDescription() {
        return childDescription;
    }

    public void setChildDescription(String childDescription) {
        this.childDescription = childDescription;
    }

    public Parent getParent() {
        return parent;
    }

    public void setParent(Parent parent) {
        this.parent = parent;
    }

    @Override
    public String toString() {
        StringBuffer toStringValue = new StringBuffer().append("Child[");
        toStringValue.append(parentId).append('.');
        toStringValue.append(childId).append('.');
        toStringValue.append(childDescription).append(']');
        return toStringValue.toString();
    }
}

package com.jpa.test;

import java.io.Serializable;

public class ChildPK implements Serializable {

    private static final long serialVersionUID = 3087264008385776228L;

    public int parentId;
    public int childId;

    public ChildPK() {
    }

    public ChildPK(int parentId, int childId) {
        this.parentId = parentId;
        this.childId = childId;
    }

    public int getParentId() {
        return parentId;
    }

    public void setParentId(int parentId) {
        this.parentId = parentId;
    }

    public int getChildId() {
        return childId;
    }

    public void setChildId(int childId) {
        this.childId = childId;
    }

    @Override
    public int hashCode() {
        int hashCode = 0;
        hashCode += parentId;
        hashCode += childId;
        return hashCode;
    }

    @Override
    public boolean equals(Object obj) {
        if (!(obj instanceof ChildPK)) {
            return false;
        }
        ChildPK pk = (ChildPK) obj;
        boolean eq = true;
        if (obj == null) {
            eq = false;
        } else {
            eq = eq && parentId == pk.parentId;
            eq = eq && childId == pk.childId;
        }
        return eq;
    }

    @Override
    public String toString() {
        StringBuffer toStringValue = new StringBuffer().append('[');
        toStringValue.append(parentId).append('.');
        toStringValue.append(childId).append(']');
        return toStringValue.toString();
    }
}

package com.jpa.test;

import java.util.List;
import javax.persistence.*;
import org.apache.log4j.*;

public class Test {

    /**
     * Log4j Logger for this class
     */
    private static final Logger log = Logger.getLogger(Test.class);

    public void doStuff() {
        log.debug("inside doStuff()");

        // create entity manager
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("Scratchpad");
        EntityManager em = emf.createEntityManager();
        log.debug("created EntityManager");

        // start transaction
        EntityTransaction transaction = em.getTransaction();
        transaction.begin();
        log.debug("started transaction");

        Parent p1 = new Parent(1, "Parent one");
        em.persist(p1);

        Child c1 = new Child(p1, 1, "Child one of parent one");
        em.persist(c1);

        Child c2 = new Child(p1, 2, "Child two of parent one");
        em.persist(c2);

        // commit transaction
        transaction.commit();
        log.debug("committed transaction");

        // start transaction
        transaction = em.getTransaction();
        transaction.begin();
        log.debug("started transaction");

        List<Parent> result1 = em.createQuery("SELECT p FROM Parent p").getResultList();
        log.debug("got " + result1.size() + " results");
        Parent parent = null;
        for (Parent p : result1) {
            parent = p;
            log.debug(p + " " + p.getChildren());
        }

        List<Child> result2 = em.createQuery("SELECT c FROM Child c").getResultList();
        log.debug("got " + result2.size() + " results");
        for (Child c : result2) {
            log.debug(c + " " + c.getParent());
        }

        // commit transaction
        transaction.commit();
        log.debug("committed transaction");
    }

    public static void main(String[] args) {
        PropertyConfigurator.configure("log4j.properties");
        Test m = new Test();
        m.doStuff();
    }
}