Here is the entity relationship diagram of the old, single table structure:

And here is the entity relationship diagram of the new, two table structure:

As you can see, it is a very simple example of a very common relationship in the database. However, what we found when implementing this in Hibernate was not a simple as I had hoped.

To get started here is the SQL used to represent our new tables:

CREATE TABLE Listing
(
id BIGINT(20) NOT NULL AUTO_INCREMENT,
price DECIMAL(10,2),
PRIMARY KEY (id)
) TYPE = INNODB;

CREATE TABLE ListingLocation
(
listingID BIGINT(20) NOT NULL,
address VARCHAR(255),
PRIMARY KEY(listingID),
INDEX (listingID),
FOREIGN KEY (listingID) REFERENCES Listing (id)
) TYPE = INNODB;


I’ve used MySQL for this example because it is free and easy to setup. InnoDB table types have been because they allow foreign keys, which is crucial to this example.

Our old Listing entity was pretty basic; it looked something like this (imports & getters/setters excluded):

@Entity
@Table
public class Listing implements Serializable {
@Id
@GeneratedValue
private long id;
@Column(columnDefinition= "DECIMAL(10,2)")
private double price;
private String address;
...

In this case, creating a new instance of Listing and persisting it is very easy. When we split the entities, it becomes a little more complicated. Here is what our entities looked like after being split:

@Entity
@Table
public class Listing implements Serializable {
@Id
@GeneratedValue
private long id;
@Column(columnDefinition= "DECIMAL(10,2)")
private double price;
@OneToOne
@PrimaryKeyJoinColumn
private ListingLocation listingLocation;
...

and

@Entity
@Table
public class ListingLocation implements Serializable {
@Id
@Column(name = "listingID")
private Long id;
private String address;
...


The differences are not large, but there are a couple important points to note:

• Adding ListingLocation to Listing with @OneToOne & @PrimaryKeyJoinColumn annotations tells Hibernate the Listing has a one-to-one mapping with ListingLocation by using the Primary Key as the join column.
• Adding @Id & @Column(name = “listingID”) annotations to our id field in ListingLocation tells Hibernate that id is, well, an ID, but that column should not be called “id” in the DB, but “listingID” as that will help an observer see the relationship quickly without looking closely at the schema. Also, it is good to note that the @GeneratedValue is not on “id” in ListingLocation as it is in Listing as we want to specify exactly what goes in that field.

The biggest gripe I have with using the one-to-one relationship is that we can no longer save Listing only. Our REAL Listing entity is far more complex with several relationships, but this was the first one-to-one relationship with Hibernate. Previously we could do something like:

Listing listing = new Listing();
listing.setPrice(price);
listing.setAddress(address);
listing.setFoo(foo); // where foo is a @OneToMany annotated entity
...
session.save(listing);

Now, we must save Listing and ListingLocation separately like this:

ListingLocation listingLocation = new ListingLocation();
listingLocation.setAddress(address);
Listing listing = new Listing();
listing.setPrice(price);
listing.setListingLocation(listingLocation);
...
session.save(listing);
if (listing.getListingLocation() != null) {
listing.getListingLocation().setId(listing.getId());
session.save(listing.getListingLocation()); // save ListingLocation
}


I guess I’m just a bit spoiled, but I was hoping that this would bit more automatic, as it is with the one-to-many/many-to-one relationships.

I have written a small app that uses these two entities and inserts a row into each table, the link is available at the bottom of this post. The config (hibernate.cfg.xml) expects that you have MySQL running on 127.0.0.1 with a DB named ‘OneToOneDemo’ with a user named ‘root’ and a password of ‘password’; I have included the SQL (Setup-OneToOneDemo.sql) to setup the DB. Just extract the contents of the archive, navigate to the project directory, and from the command line/terminal run:

mvn clean compile exec:java -Dexec.mainClass=com.point2.onetoonedemo.App -e


You should see the following output:

@OneToOne @PrimaryKeyJoinColumn Hibernate Demo
----------------------------------------------
...
Hibernate:
insert into Listing (price) values (?)

Hibernate:
insert into ListingLocation (address, listingID) values (?, ?)
Saved listing ID: 1


Download Source Code (12 KB .ZIP)

If you have any questions, comments, or suggestions on how to do better accomplish one-to-one Hibernate mappings, I would love to hear about them. If you have any problems getting the code to compile and/or run, please let me know and I will make the necessary changes. Everything should just work, providing you modify the hibernate config or setup your DB. I had a difficult time finding complete and recent documentation on this subject so I hope this post and the maven project will help.

By: Damien Gabrielson

For developers who have been using Maven for a while, you likely will be creating your projects (.pom files specifically) from a text editor but this can be a little overwhelming for those new to Maven or those who would rather simply spend their time writing code instead. NetBeans 6.5 makes it incredibly easy to create a Maven project in a matter of seconds.

First, go to the “File” menu and choose “New Project”.

Under “Categories”, choose “Maven” and in”Projects”, choose “Maven Project” and click “Next”.

The “Archetype” window should open next, select “Maven Quickstart Archetype” and click “Next”.

The “Name and Location” window will open allowing you to give specifics to your project. You can customize this as you see fit but for the purposes of this example I will leave the default settings. Click “Finish” and your project will start to build.

If you are running OS X 10.5.6, as I am, you will likely see the following error:

------------------------------------------------------------------------
[ERROR]BUILD ERROR
------------------------------------------------------------------------
Error resolving version for 'org.apache.maven.plugins:maven-archetype-plugin': Plugin requires Maven version 2.0.7
------------------------------------------------------------------------
For more information, run Maven with the -e switch


The reason for this is that OS X 10.5 shipped with Maven 2.0.6 pre-installed but Apple has never pushed any updates to it. According to the Apache archives, Maven 2.0.6 was released in April 2007 but it is kind of annoying that they didn’t realize that Maven projects would not work by default on OS X before they released NetBeans 6.5. Luckily, it is very easy to update Maven to the latest version if you don’t mind using the terminal and the sudo command.

1. Download the zip with binaries for whichever Maven version you would like to install. At time of writing, the most recent version is 2.1.0, http://www.apache.org/dyn/closer.cgi/maven/binaries/apache-maven-2.1.0-bin.zip.
2. Once the file is downloaded, you can extract the archive and you should end up with a directory named something like “apache-maven-2.1.0″. Open a Terminal window and cd /usr/share
3. Let’s move the current Maven directory to a backup folder in case we ever want to revert. In the terminal you opened in the last step, type sudo mv maven maven-2.0.6. This will ask you for your password and then move the “maven” directory to a new directory called “maven-2.0.6″, assuming your user account has sufficient permissions.
4. Now let’s put the new version in place so that NetBeans will start using it. To do so, we need to copy the directory we extracted from the zip file. Once again, in the Terminal window you have open, type sudo cp -R /Users/dgabrielson/Desktop/apache-maven-2.1.0 maven. Keep in mind that the path to the extracted “apache-maven-2.1.0″ directory will have to be changed to the appropriate path on your system.
5. Maven should now be upgraded! To confirm, in your Terminal window, type mvn -version and you should see some output like Apache Maven 2.1.0 (r755702; 2009-03-18 13:10:27-0600).

Now if you try to build again, you should get a successful build log ending with:

------------------------------------------------------------------------
BUILD SUCCESSFUL
------------------------------------------------------------------------


Enjoy your new Maven build and get coding!

By: Damien Gabrielson

• Ensure that JDK 1.6 and NetBeans 6.5 are installed
• Locate the NetBeans config file, typically located at “/Applications/NetBeans/NetBeans 6.5.app/Contents/Resources/NetBeans/etc/netbeans.conf” and open the file in your favourite editor
• Set the “netbeans_jdkhome” parameter to the JDK 1.6 home directory (ie. netbeans_jdkhome=
  /System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home)
• In NetBeans, right click on the project in the project window and select “Properties”
• Ensure the “Source/Binary Format” is set to “1.6″
  
• Click on “Tools” on the toolbar and select “Java Platforms”
• Under “J2SE” the default platform should now be “JDK 1.6″
  

By: Damien Gabrielson