How to Set Primary / Foreign Keys or Relationships with Hibernate 3 / JPA Tables?

[James Dekker]

Hello there...

Created 3 JPA annotated classes:

User:

@Entity
@Table(name = "user", schema = "test")
public class User {
	/* Data Fields */
	private int userid;
	private String username;
	private String password;

	public User() { }

	@Id
	@GeneratedValue(strategy = IDENTITY)
	@Column(name = "userid", unique = true, nullable = false)
	public Integer getUserid() {
		return this.userid;
	}

	public void setUserid(Integer userid) {
		this.userid = userid;
	}

	@Column(name = "username", nullable = false, length = 45)
	public String getUsername() {
		return this.username;
	}

	public void setUsername(String username) {
		this.username = username;
	}

	@Column(name = "password", nullable = false, length = 45)
	public String getPassword() {
		return this.password;
	}

	public void setPassword(String password) {
		this.password = password;
	}

	@Override
	public String toString() {
		return  "ID: " + getUserid() + "/n" + 
		        "Userame: " + getUsername() + "/n" + 
		        "Password: " + getPassword();
	}
}

Device:

@Entity
@Table(name = "device", schema = "test")
public class Device {
	private int id;
	private int uuid;

	@Id
	@GeneratedValue
	@Column(name = "id", unique = true, nullable = false)
	public int getId() {
		return id;
	}

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

	@Column(name = "uuid")
	public int getUuid() {
		return uuid;
	}

	public void setUuid(int uuid) {
		this.uuid = uuid;
	}

	@Override
	public String toString() {
		return "ID: " + getId() + "/n" + 
		       "UUID: " + getUuid();
	}
}

Token:

@Entity
@Table(name = "token", schema = "test")
public class Token {
	private long id;
	private Boolean expired;
	private String value;
	private Date creationTime;
	private Date expirationTime;

	@Id
	@GeneratedValue
	@Column(name = "id", unique = true, nullable = false)
	public long getId() {
		return id;
	}

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

	@Basic
	public Boolean isExpired() {
		return expired;
	}

	public void setExpired(Boolean expired) {
		this.expired = expired;
	}

	@Column(name = "value")
	public String getValue() {
		return value;
	}

	public void setValue(String value) {
		this.value = value;
	}

	@Temporal(TemporalType.TIME)
	public Date getCreationTime() {
		return creationTime;
	}

	public void setCreationTime(Date creationTime) {
		this.creationTime = creationTime;
	}

	@Temporal(TemporalType.TIME)
	public Date getExpirationTime() {
		return expirationTime;
	}

	public void setExpirationTime(Date expirationTime) {
		this.expirationTime = expirationTime;
	}

	@Override
	public String toString() {
		return "ID: " + getId() + "/n" +
			   "Value: " + getValue() + "/n" +
			   "Creation: " + getCreationTime() + "/n" +
			   "Expiration: " + getExpirationTime() + "/n" +
			   "Expired: " + isExpired();
	}
}

Am trying to set up ER relationships in Hibernate 3 / JPA (or even in MySQL 5) like this:

User has one to many Devices.

User.userid (Primary Key)
Device.id (Primary Key)

Device has one to one relationship with Token.

Device.id (Primary Key)
Token.id (Primary Key)

Question(s):

(1) How do I set these up in Hibernate3 or MySQL5 code?

(2) Am very new with SQL so would it be that the Device.id is the foreign key of User.userid and vice versa with Device and Token?

Am very new to Hibernate 3 / JPA so would appreciate it if someone could help...

Thank you and happy programming!

[Cameron Wallace McKenzie]

So, you'll need instance variables:

User has one to many Devices.

So, the User class will has a List of devices, List devices = new ArrayList(); Put in setters and getters. Device will have an instance of a User. Then add your JPA annotations like usual.

Device has one to one relationship with Token.

Add instance variables - Devices has an instance of a Token, and a Token has an instance of a Device. Then perform the annotation mapping as usual with any one-to-one association.

Here's a good tutorial on mapping one-to-one relations with Hibernate and JPA:

Mapping Associations with the Java Persistence API

There's a discussion of one-to-many mappings on that site as well.

Keep asking questions, and let us know how you're getting along, including code snippets and any errors you have.

-Cameron McKenzie

[James Dekker]

Cameron,

Yes, I checked out the tutorial and got my code working!

The tutorial does throw an exception however...

Created my tables like this:

CREATE TABLE examscam.exam (
	id int(11) NOT NULL auto_increment,
	shortName varchar(255) default NULL,
	detail_id int(11) default NULL,
	PRIMARY KEY (id), 
	KEY FK2FB81FB83A97F5 (detail_id),
	CONSTRAINT FK2FB81FB83A97F5
	FOREIGN KEY (detail_id)
	REFERENCES exam_detail (id)
)

CREATE TABLE examscam.exam_detail (
	id int(11) NOT NULL auto_increment,
	fullName varchar(255) default NULL,
	numberOfQuestions int(11) NOT NULL default 0,
	passingPercentage int(11) NOT NULL default 0,
	PRIMARY KEY (id) 
)

Exam:

@Entity
@Table(name = "exam", schema = "examscam")
public class Exam {

	private int id;
	private String shortName;
	private ExamDetail detail;

	@Id
	@GeneratedValue
	@Column(name="id")
	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}
	
	@Basic
	public String getShortName() {
		return shortName;
	}

	public void setShortName(String shortName) {
		this.shortName = shortName;
	}
	
	@OneToOne(cascade = CascadeType.ALL, 
			  fetch = FetchType.LAZY)
	@JoinColumn(name="detail_id")
	public ExamDetail getDetail() {
		return detail;
	}

	public void setDetail(ExamDetail detail) {
		this.detail = detail;
	}
}

ExamDetail:

@Entity
@Table(name = "exam_detail", schema = "examscam")
public class ExamDetail {

	private int id;
	private String fullName;
	private int numberOfQuestions;
	private int passingPercentage;
	private Exam exam;

	@Id
	@GeneratedValue
	@Column(name="id")
	public int getId() {
		return id;
	}

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

	@Basic
	public String getFullName() {
		return fullName;
	}

	public void setFullName(String fullName) {
		this.fullName = fullName;
	}
	
	@Basic
	public int getNumberOfQuestions() {
		return numberOfQuestions;
	}

	public void setNumberOfQuestions(int numberOfQuestions) {
		this.numberOfQuestions = numberOfQuestions;
	}

	@Basic
	public int getPassingPercentage() {
		return passingPercentage;
	}

	public void setPassingPercentage(int passingPercentage) {
		this.passingPercentage = passingPercentage;
	}

	@OneToOne(mappedBy="detail", cascade=CascadeType.ALL)
	public Exam getExam() {
		return exam;
	}

	public void setExam(Exam exam) {
		this.exam = exam;
	}
}

ExamApp:

package com.examscam.exam;

import org.hibernate.Session;

import com.examscam.HibernateUtil;

public class ExamApp {
	
	public static void main(String[] args) {
		Session session = HibernateUtil.beginTransaction();
		HibernateUtil.recreateDatabase();
		Exam exam = new Exam();
		exam.setShortName("SCEA");
		
		ExamDetail detail = new ExamDetail();
		detail.setFullName("Sun Certified Enterprise Architect");
		detail.setPassingPercentage(62);
		detail.setNumberOfQuestions(55);
		
		exam.setDetail(detail);
		
		session.save(exam);
		HibernateUtil.commitTransaction();
	}
}

When you run ExamApp this is what happens:

21:45:23,800  INFO SchemaExport:268 - schema export complete
21:45:23,802  INFO DriverManagerConnectionProvider:170 - cleaning up connection pool: jdbc:mysql://localhost/examscam
Exception in thread "main" org.hibernate.MappingException: Unknown entity: com.examscam.exam.Exam
	at org.hibernate.impl.SessionFactoryImpl.getEntityPersister(SessionFactoryImpl.java:628)
	at org.hibernate.impl.SessionImpl.getEntityPersister(SessionImpl.java:1366)
	at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:121)

Question(s):

(1) In the CreateExam.sql query, you stated to place this:

KEY FK2FB81FB83A97F5 (detail_id),
CONSTRAINT FK2FB81FB83A97F5

What is the FK2FB81FB83A97F5 for and where do I create values like this for my own foreign keys?

(2) What is causing the exception?

Thank you for everything!

-James

[Cameron Wallace McKenzie]

Well, in this case, you just forgot to add the Exam and ExamDetail to the HibernateConfig in the HibernateUtil class:

  
public static Configuration getInitializedConfiguration(){
  AnnotationConfiguration config 
              = new AnnotationConfiguration();
/* add all of your JPA annotated classes here!!!*/
  config.addAnnotatedClass(User.class);
/*Future classes we will be creating. 
Keep them commented out for now.*/
  config.addAnnotatedClass(Exam.class);
  config.addAnnotatedClass(ExamDetail.class);

  config.configure();
  return config;
}  

Creating a HibernateUtil Class

-Cameron McKenzie

[James Dekker]

Cameron,

Thank you for your help! You stated these fields in your tutorial:

KEY FK2FB81FB83A97F5 (detail_id),
CONSTRAINT FK2FB81FB83A97F5

What is the FK2FB81FB83A97F5 for and where do I create values like this for my own foreign keys?

-James

[Cameron Wallace McKenzie]

Oh, I have no idea what those are. I think it's just the identifier MySQL gives to the foreign key constraint. With the various classes added to the Hibernate Config, you can have Hibernate create your database tables for you.

    
public static void main(String args[]) {
    AnnotationConfiguration config = 
                 new AnnotationConfiguration();
    config.addAnnotatedClass(User.class);
    config.configure();
    new SchemaExport(config).create(true, true);
  }

Creating Tables with the SchemaExport create

-Cameron McKenzie

[James Dekker]

Okay, cool... Thank you! I've used the SchemaExport mechanism before but still felt safer by creating tables in MySQL by using the scripts.

-James

[Cameron Wallace McKenzie]

The nice thing is that the log files actually show you the scripts. In fact, I think if you do create(true,false) or something like that, it spits out the script in the log file, but doesn't actually connect to the database and create the tables, so you can see exactly what SQL, and it is SQL, that Hibernate will use. It's a great feature if you can do a 'top down' database design.

-Cameron McKenzie