Relationship Example

Create a java project "sawan.modi.jpa.eclipselink", create again a folder "lib" and place the required JPA jars and derby.jar into this folder.

Create the package "sawan.modi.jpa.eclipselink.model" and the following classes.

   
package sawan.modi.jpa.eclipselink.model;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;

@Entity
public class Family {
 @Id
 @GeneratedValue(strategy = GenerationType.TABLE)
 private int id;
 private String description;

 @OneToMany(mappedBy = "family")
 private final List members = new ArrayList();

 public int getId() {
  return id;
 }

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

 public String getDescription() {
  return description;
 }

 public void setDescription(String description) {
  this.description = description;
 }

 public List getMembers() {
  return members;
 }

}

  

   
package sawan.modi.jpa.eclipselink.model;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import javax.persistence.Transient;

@Entity
public class Person {
 @Id
 @GeneratedValue(strategy = GenerationType.TABLE)
 private String id;
 private String firstName;
 private String lastName;

 private Family family;

 private String nonsenseField = "";

 private List jobList = new ArrayList();

 public String getId() {
  return id;
 }

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

 public String getFirstName() {
  return firstName;
 }

 public void setFirstName(String firstName) {
  this.firstName = firstName;
 }

 // Leave the standard column name of the table
 public String getLastName() {
  return lastName;
 }

 public void setLastName(String lastName) {
  this.lastName = lastName;
 }

 @ManyToOne
 public Family getFamily() {
  return family;
 }

 public void setFamily(Family family) {
  this.family = family;
 }

 @Transient
 public String getNonsenseField() {
  return nonsenseField;
 }

 public void setNonsenseField(String nonsenseField) {
  this.nonsenseField = nonsenseField;
 }

 @OneToMany
 public List getJobList() {
  return this.jobList;
 }

 public void setJobList(List nickName) {
  this.jobList = nickName;
 }

}
  

   
package sawan.modi.jpa.eclipselink.model;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Job {
 @Id
 @GeneratedValue(strategy = GenerationType.TABLE)
 private int id;
 private double salery;
 private String jobDescr;

 public int getId() {
  return id;
 }

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

 public double getSalery() {
  return salery;
 }

 public void setSalery(double salery) {
  this.salery = salery;
 }

 public String getJobDescr() {
  return jobDescr;
 }

 public void setJobDescr(String jobDescr) {
  this.jobDescr = jobDescr;
 }

}

  

Create the file "persistence.xml" in "src/META-INF". Remember to change the path to the database.

   

 xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
 version="2.0" xmlns="http://java.sun.com/xml/ns/persistence">
 
  
  
  sawan.modi.jpa.eclipselink.model.Person
  sawan.modi.jpa.eclipselink.model.Family
  sawan.modi.jpa.eclipselink.model.Job
  
  
   
        value="jdbc:derby:/home/vogella/databases/relationsshipDb;create=true" />
   
   

   
   
       value="database" />
  

 



  

The following check is implemented as a JUnit Test. For details please see See JUnit Tutorial . The setup() method will create a few test entries. After the test entries are created, they will be read and the one field of the entries is changed and saved to the database.

   
package sawan.modi.jpa.eclipselink.main;

import static org.junit.Assert.assertTrue;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;

import org.junit.Before;
import org.junit.Test;

import sawan.modi.jpa.eclipselink.model.Family;
import sawan.modi.jpa.eclipselink.model.Person;

public class JpaTest {

 private static final String PERSISTENCE_UNIT_NAME = "people";
 private EntityManagerFactory factory;

 @Before
 public void setUp() throws Exception {
  factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
  EntityManager em = factory.createEntityManager();

  // Begin a new local transaction so that we can persist a new entity
  em.getTransaction().begin();

  // Read the existing entries
  Query q = em.createQuery("select m from Person m");
  // Persons should be empty

  // Do we have entries?
  boolean createNewEntries = (q.getResultList().size() == 0);

  // No, so lets create new entries
  if (createNewEntries) {
   assertTrue(q.getResultList().size() == 0);
   Family family = new Family();
   family.setDescription("Family for the Knopfs");
   em.persist(family);
   for (int i = 0; i < 40; i++) {
    Person person = new Person();
    person.setFirstName("Jim_" + i);
    person.setLastName("Knopf_" + i);
    em.persist(person);
    // First we have to persists the job
    // Now persists the new person
    family.getMembers().add(person);
    em.persist(person);
    em.persist(family);
   }
  }

  // Commit the transaction, which will cause the entity to
  // be stored in the database
  em.getTransaction().commit();

  // It is always good practice to close the EntityManager so that
  // resources are conserved.
  em.close();

 }

 @Test
 public void checkAvailablePeople() {

  // Now lets check the database and see if the created entries are there
  // Create a fresh, new EntityManager
  EntityManager em = factory.createEntityManager();

  // Perform a simple query for all the Message entities
  Query q = em.createQuery("select m from Person m");

  // We should have 40 Persons in the database
  assertTrue(q.getResultList().size() == 40);

  em.close();
 }

 @Test
 public void checkFamily() {
  EntityManager em = factory.createEntityManager();
  // Go through each of the entities and print out each of their
  // messages, as well as the date on which it was created
  Query q = em.createQuery("select f from Family f");

  // We should have one family with 40 persons
  assertTrue(q.getResultList().size() == 1);
  assertTrue(((Family) q.getSingleResult()).getMembers().size() == 40);
  em.close();
 }

 @Test(expected = javax.persistence.NoResultException.class)
 public void deletePerson() {
  EntityManager em = factory.createEntityManager();
  // Begin a new local transaction so that we can persist a new entity
  em.getTransaction().begin();
  Query q = em
    .createQuery("SELECT p FROM Person p WHERE p.firstName = :firstName AND p.lastName = :lastName");
  q.setParameter("firstName", "Jim_1");
  q.setParameter("lastName", "Knopf_!");
  Person user = (Person) q.getSingleResult();
  em.remove(user);
  em.getTransaction().commit();
  Person person = (Person) q.getSingleResult();
  // Begin a new local transaction so that we can persist a new entity

  em.close();
 }
}

  

The project should now look like the following.

You should be able to run the JUnit tests successfully .