Does Hibernate support optional one-to-one associations?

ilchen · **Posted:** Tue Aug 15, 2006 5:40 am

Hello,

I am trying to figure out if Hibernate supports optional one-to-one associations, e.g. Person (0..1) -- (0..1) House and so far am not finding anything.

The Hibernate books and reference all talk of two ways of mapping one-to-one associations:
1. using a many-to-one fk association with a unique constraint on the fk;
2. using a pk association.

the trouble with both of them is that they enforce the mandatory property of an association end, which sometimes is not desirable. One enforces the mandatory property with the uniqueness constraint on the fk, the other using the fact that one table's pk is its fk.

I tried to see if I could adapt the 1st way of representing one-to-ones to
support optional association ends. What I did was simply remove the unique and not null constrains from DDL generated (without removing it it would be impossible to have a Person record that doesn't reference a passport record):

from

Code:

CREATE TABLE Person (
PassUniqueId                   VARCHAR (32)                NOT NULL,
version                        INTEGER                       NOT NULL,
uniqueId                       VARCHAR (32)                   NOT NULL,
name                           VARCHAR (40)                   NULL,
PRIMARY KEY (uniqueId),
UNIQUE(PassUniqueId),
FOREIGN KEY (PassUniqueId) REFERENCES Passport (uniqueId)
);

to

Code:

CREATE TABLE Person (
PassUniqueId                   VARCHAR (32)                NULL,
version                        INTEGER                       NOT NULL,
uniqueId                       VARCHAR (32)                   NOT NULL,
name                           VARCHAR (40)                   NULL,
PRIMARY KEY (uniqueId),
FOREIGN KEY (PassUniqueId) REFERENCES Passport (uniqueId)
);

but leave the uniqueness attribute in the mapping file:

Code:

        <many-to-one
            name="pass"
            class="application.business.logic.Passport"
            cascade="save-update,merge"
            unique="true"
        >
            <column name="PassUniqueId"/>
        </many-to-one>

The resulting solution worked fine and I was able to create instances of Person not linked to a Passport. However Hibernate didn't succed in keeping the association from degrading into a many-to-one. It enabled me to create two Persons linked with the same Passport:

Code:

 PASSUNIQUEID    | VIRSION | UNIQUEID    | Name
================================================
 0d8b3919fffff   | 1       | 0d5fb1dcfff | John
 0d8b3919fffff   | 0       | 10fd07bdfff | Sam

A subsequent attempt at retrieving such Persons with Hibernate failed:
org.hibernate.HibernateException: More than one row with the given identifier was found: 0d8b3919ffffffd60151e135b6da0164, for class: application.business.logic.Passport.

Is there a way to configure hibernate to keeping the association from degrading into a many-to-one without relying on an RDBMS engine?

Hibernate version: 3.2.CR2

Thank you,
Andrei.

damnhandy · **Posted:** Tue Aug 15, 2006 8:33 am

If you use a PK association so that your Passport shares the same PK as your Person instance, you should use the FK ID generator in the Passport mapping:

Code:

<id name="passportId">
  <column name="PassId" />
         <generator class="foreign"> 
               <param name="property">person</param> 
          </generator> 
 </id>

<one-to-one name="person" class="Person" constrained="true" />  

The Passport mapping uses the person reference to derive it's PK. Then, in your Person mapping, you continue to use the many-to-one association, but adding a not-found="ignore" attribute so that it won't throw an exception when a passport is not found for Persons without a passport:

Code:

<many-to-one
            name="pass"
            class="application.business.logic.Passport"
            cascade="save-update,merge"
            unique="true"
            not-found="ignore"
        >
            <column name="PassportId"/>
        </many-to-one>

I have found not-found="ignore" to be non-performant as it disbaled batch-fetching on the related entity. There are two ways I get around that:

Code:

<many-to-one
            name="pass"
            class="application.business.logic.Passport"
            cascade="save-update,merge"
            unique="true"
            formula="(select p.passportId from Passport where p.personId = personId)"
        >
        </many-to-one>

Or, use fetch="join"

Code:

<many-to-one
            name="pass"
            class="application.business.logic.Passport"
            cascade="save-update,merge"
            unique="true"
            fetch="join"
            not-found="ignore"
        >
            <column name="PassportId"/>
        </many-to-one>

Hope this helps.

Ryan-

ilchen · **Posted:** Tue Aug 15, 2006 8:51 am

Ryan, thank you very much for your time and the reply. Our problem is that we cannot use the pk mapping (Passport shares the same pk instance as Person). We really need to figure out a way to get optional one-to-one associations to work with the fk mapping. My mapping file for passport looks as follows:
<class
name="application.business.logic.Passport"
table="Passport"
optimistic-lock="version"
mutable="true"
>

<id
name="id"
column="uniqueId"
type="string">
<generator class="assigned"/>
</id>

...
...


<one-to-one
name="person"
class="application.business.logic.Person"
cascade="save-update,merge"
constrained="false"
property-ref="pass"
>

As I mentioned previously we got everything to work with the many-to-one fk association. The main trouble is that we discover that Hibernate puts no effort into ensuring that the association doesn't degrade into a many-to-one and seems to rely soly on the RDBMS and the UNIQUE DDL constraint for that matter.

ilchen · **Posted:** Wed Aug 16, 2006 8:42 am

FYI: I got convinced that optional one-to-one associations are simply not properly supported in Hibernate and created a bug report for it: http://opensource.atlassian.com/projects/hibernate/browse/HHH-2007.