Hibernate one-to-one mapping using annotations

If you are working on any hibernate project or you are planning to work on any in future, then you can easily understand the one-to-one relationships between several entities in your application. In this post, i will discuss variations of one-to-one mappings supported in hibernate.
Download source code

Sections in this post:
Various supported techniques
Using foreign key association
Using a common join table
Using shared primary key

For this article, I am extending the example written for hello world example. We have two entities here: Employee and Account.

Various supported techniques

In hibernate there are 3 ways to create one-to-one relationships between two entities. Either way you have to use @OneToOne annotation. First technique is most widely used and uses a foreign key column in one to table. Second technique uses a rather known solution of having a third table to store mapping between first two tables. Third technique is something new which uses a common primary key value in both the tables.
Lets see them in action one by one:

Using foreign key association

In this association, a foreign key column is created in owner entity. For example, if we make EmployeeEntity owner, then a extra column “ACCOUNT_ID” will be created in Employee table. This column will store the foreign key for Account table.
Table structure will be like this:

To make such association, refer the account entity in EmployeeEntity class as follow:

@OneToOne

@JoinColumn(name="ACCOUNT_ID")

private AccountEntity account;

The join column is declared with the @JoinColumn annotation which looks like the @Column annotation. It has one more parameters named referencedColumnName. This parameter declares the column in the targeted entity that will be used to the join.
If no @JoinColumn is declared on the owner side, the defaults apply. A join column(s) will be created in the owner table and its name will be the concatenation of the name of the relationship in the owner side, _ (underscore), and the name of the primary key column(s) in the owned side.
In a bidirectional relationship, one of the sides (and only one) has to be the owner: the owner is responsible for the association column(s) update. To declare a side as not responsible for the relationship, the attribute mappedBy is used. mappedBy refers to the property name of the association on the owner side.

1.@OneToOne(mappedBy="account")

2.private EmployeeEntity employee;

Above “mappedBy” attribute declares that it is dependent on owner entity for mapping.
Lets test above mappings in running code:

 01.public class TestForeignKeyAssociation {  
 03.  public static void main(String[] args) {  
 04.    Session session = HibernateUtil.getSessionFactory().openSession();  
 05.    session.beginTransaction();  
 06.   
 07.    AccountEntity account = new AccountEntity();  
 08.    account.setAccountNumber("123-345-65454");  
 09.   
 10.    // Add new Employee object  
 11.    EmployeeEntity emp = new EmployeeEntity();  
 12.    emp.setEmail("demo-user@mail.com");  
 13.    emp.setFirstName("demo");  
 14.    emp.setLastName("user");  
 15.   
 16.    // Save Account  
 17.    session.saveOrUpdate(account);  
 18.    // Save Employee  
 19.    emp.setAccount(account);  
 20.    session.saveOrUpdate(emp);  
 21.   
 22.    session.getTransaction().commit();  
 23.    HibernateUtil.shutdown();  
 24.  }  
 25.}

Running above code creates desired schema in database and run these SQL queries.

1.Hibernate: insert into ACCOUNT (ACC_NUMBER) values (?)

2.Hibernate: insert into Employee (ACCOUNT_ID, EMAIL, FIRST_NAME, LAST_NAME) values (?, ?, ?, ?)

You can verify the data and mappings in both tables when you run above program..

Using a common join table

This approach is not new to all of us. Lets start with targeted DB structure in this technique.

In this technique, main annotation to be used is @JoinTable. This annotation is used to define the new table name (mandatory) and foreign keys from both of the tables. Lets see how it is used:

@OneToOne(cascade = CascadeType.ALL)

@JoinTable(name="EMPLOYEE_ACCCOUNT", joinColumns = @JoinColumn(name="EMPLOYEE_ID"),

inverseJoinColumns = @JoinColumn(name="ACCOUNT_ID"))

private AccountEntity account;

@JoinTable annotation is used in EmployeeEntity class. It declares that a new table EMPLOYEE_ACCOUNT will be created with two columns EMPLOYEE_ID (primary key of EMPLOYEE table) and ACCOUNT_ID (primary key of ACCOUNT table).
Testing above entities generates following SQL queries in log files:

Hibernate: insert into ACCOUNT (ACC_NUMBER) values (?)

Hibernate: insert into Employee (EMAIL, FIRST_NAME, LAST_NAME) values (?, ?, ?)

Hibernate: insert into EMPLOYEE_ACCCOUNT (ACCOUNT_ID, EMPLOYEE_ID) values (?, ?)

Using shared primary key

In this technique, hibernate will ensure that it will use a common primary key value in both the tables. This way primary key of EmployeeEntity can safely be assumed the primary key of AccountEntity also.
Table structure will be like this:

In this approach, @PrimaryKeyJoinColumn is the main annotation to be used.Let see how to use it.

@OneToOne(cascade = CascadeType.ALL)

@PrimaryKeyJoinColumn

private AccountEntity account;

In AccountEntity side, it will remain dependent on owner entity for the mapping.

1.@OneToOne(mappedBy="account", cascade=CascadeType.ALL)

2.private EmployeeEntity employee;

Testing above entities generates following SQL queries in log files:

1.Hibernate: insert into ACCOUNT (ACC_NUMBER) values (?)

2.Hibernate: insert into Employee (ACCOUNT_ID, EMAIL, FIRST_NAME, LAST_NAME) values (?, ?, ?, ?)

So, we have seen all 3 types of one to one mappings supported in hibernate. I will suggest you to download the source code and play with it.
Happy Learning !!

Download source code

Source: http://howtodoinjava.com/2012/11/15/hibernate-one-to-one-mapping-using-annotations/

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Wednesday, 19 February 2014