Hibernate one-to-one (on foreign key) vs one-to-one (on primary key)

Question

I have questions about
What are the benefits of using a one-to-one association on foreign key or a one-to-one association on the primary key?

I've read the documentation of Hibernate available at: http://docs.jboss.org/hibernate/orm/3.3/reference/en/html/associations.html#assoc-unidirectional-121

Can someone tell me with details the plus of using the first or the second implementation?

Another question, I have a one-to-one unidirectional relationship between User and Perimeter.
(User ----> Perimeter) and I want to use the association based on the foreign key.
Could I reverse the direction into (User <---- Perimeter) so the table User stay intact?
I think that is not reasonable (perimeter.getUser() !!!) but it's technically possible?

score 3 · Answer 1 · answered Dec 14 '12 at 11:14

3

I see two advantages

Having the choice lets you map existing schemas, whatever they have chosen as strategy
using a foreign key in A pointing to the ID in B, Hibernate knows that there is no B for a given A if the foreign key is null. It can't know that when primary keys are used. So it's more efficient to use a foreign key if the association is optional.

For your second question, yes, of course it's possible. That is called a bidirectional OneToOne association:

public class User {
    // ...

    @OneToOne
    @JoinColumn
    private Perimeter perimeter;
}

public class Perimeter {
    // ...

    @OneToOne(mappedBy = "perimeter")
    private User user;
}

answered Dec 14 '12 at 11:14

JB Nizet

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first of all thank you for your reply, i understood the two advantages. but for my second question i don't want a bidirectional relationship, just User <-- Perimeter direction because (i refer to the xml mapping in the hibernate documentation i don't know annotations very well) bidirectional oblige me to add a column into the User table but i don't want to touch the User table – Mathis Hobden Dec 14 '12 at 11:52
Making an association bidirectional doesn't change anything to the schema. If you want the perimeter table to contain the join column instead of the user table, then simply do it. – JB Nizet Dec 14 '12 at 11:59

score 3 · Accepted Answer · answered Dec 18 '12 at 22:25

In addition to advantages mentioned by JB Nizet and thanks to another person told me that the one-to-one based on foreign key is more flexible. let's consider that i use a one-to-one on foreign key (User <--- Perimeter) , if the requirements of the application changes to need multiple Users for any Perimeter, If i shared the primary key, I'll have much more refactoring to do. But as i used the foreign key, all i do is relax the unique constraint ;)

score 1 · Answer 3 · answered May 30 '13 at 06:23

one to one relationship using primary key association

In this association one-to-one relationships occurs when one entity is related to exactly one occurrence in another entity.

here is the link for example: primary key association

one to one relationship using foreign key association

In the same example do the following changes:

In StockDetail.java

    private Integer stockDetailsId;
    //with setter and getter
    //remove stockId and it's setter and getter

in stock.hbm.xml

    <id name="stockId" type="java.lang.Integer">
                <column name="STOCK_ID" />
                <generator class="assigned" />
      </id>

in StockDetail.hbm.xml

    <id name="stockDetailsId" type="java.lang.Integer">
                <column name="STOCK_DETAILS_ID" />
                <generator class="assigned"/>
     </id>
    <many-to-one name="stock" 

    class="com.test.common.Stock" column="STOCK_ID" />

in hibernate.cfg.xml

    //this is optional property if you want to create database table's according to your hbm file's.

    <property name="hibernate.hbm2ddl.auto">create</property>

Hibernate one-to-one (on foreign key) vs one-to-one (on primary key)

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