Hibernate relations

Using hibernate to create database schema we need to provide connections between our tables. In the database tables, there must exist foreign key relationship. The main advantage of putting relation ship between objects is that we can do operation on one object, and the same operation can transfer onto the other object in the database.

Using hibernate we can put the following types of relationships:

* one-to-one
* one-to-many
* many-to-one
* many-to-many



We can distinguish two types of all above relations



* unidirectional
* bidirectional



You can easily count that all types of hibernate relationships is 7(one-to-many and many-to-one are the same). In next articles I will try to clearly explain all of this relations with easy examples.

If you don’t know how to create simple hibernate project, at the beginning  you should read my previous article: Using Hibernate persistence with Maven. 

Hibernate relation: one to one (bidirectional)

1. Database schema

2. Entity class:

import javax.persistence.*;
import java.sql.Date;

@Entity
@Table(name = "User")
public class User {

    @Id
    @GeneratedValue
    private Long id;

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

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

    @Column(name = "birthDate")
    private Date birthDate;

    @Column (name = "address")
    private  String emailAddress;

    @OneToOne(cascade = {CascadeType.ALL})
    @JoinColumn(name = "creditCardId")
    private CreditCard creditCard;

    public User() {
    }

//getters and setters

}

import javax.persistence.*;
import java.sql.Date;

@Entity
@Table(name = "CreditCard")
public class CreditCard {

    @Id
    @GeneratedValue
    private Long id;

    @Column
    private String number;

    @Column
    private Date expiredDate;

    @OneToOne(mappedBy = "creditCard")
    private User user;
    public CreditCard() {
    }

    public CreditCard(String number, Date expiredDate) {
        this.number = number;
        this.expiredDate = expiredDate;
    }


}

3. Hibernate configuration file

Put annotated classes User.java and CreditCard.java in to hibernate configuration file and mysql connection details:

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
    <session-factory>
        <!-- Database connection settings -->
        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost:3306/test</property>
        <property name="connection.username">root</property>
        <property name="connection.password">root</property>

        <!-- JDBC connection pool (use the built-in) -->
        <property name="connection.pool_size">1</property>

        <!-- SQL dialect -->
        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>

        <!-- Enable Hibernate's automatic session context management -->
        <property name="current_session_context_class">thread</property>

        <!-- Disable the second-level cache  -->
        <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>

        <!-- Echo all executed SQL to stdout -->
        <property name="hibernate.show_sql">true</property>

        <property name="hbm2ddl.auto">create-drop</property>

        <mapping class="net.hibernateApp.packt.maven.domain.User"/>
        <mapping class="net.hibernateApp.packt.maven.domain.CreditCard"/>
    </session-factory>
</hibernate-configuration>

4. Run

import net.hibernateApp.packt.maven.domain.User;
import net.hibernateApp.packt.maven.domain.UserAdditionalInfo;
import net.hibernateApp.packt.maven.hibernate.HibernateUtils;
import org.hibernate.SessionFactory;
import org.hibernate.Session;

import java.sql.Date;
import java.util.List;

public class Main {

    public static void main(String[] args) {

        //WRITE
        User user = new User();
        user.setFirstName("Christopher");
        user.setSurName("Stepniak");
        user.setBirthDate(new Date(System.currentTimeMillis()));
        user.setCreditCard(new CreditCard("cardNumber", new Date(System.currentTimeMillis())));
        saveUser(user);

        CreditCard creditCard = (CreditCard) list("CreditCard").get(0);
        System.out.println(creditCard.getUser().getFirstName());
    }

    private static List list(String sourceTable) {
        SessionFactory sf = HibernateUtils.getSessionFactory();
        Session session = sf.openSession();

        List objects = session.createQuery("from "+sourceTable).list();
        session.close();
        return objects;
    }

    
    private static User saveUser(User user){
        SessionFactory sf;
        sf = HibernateUtils.getSessionFactory();
        Session session = sf.openSession();
        session.beginTransaction();
        Long id = (Long) session.save(user);
        user.setId(id);
        session.getTransaction().commit();
        session.close();
        return user;
    }


}

Output:  Christopher

Class net.hibernateApp.packt.maven.hibernate.HibernateUtils is described here:
 Using Hibernate persistence with Maven