[hibernate-issues] [Hibernate-JIRA] Commented: (HHH-2007) Hibernate doesn't support optional one-to-one associations

[Chris Wilson (JIRA)]

    [ http://opensource.atlassian.com/projects/hibernate/browse/HHH-2007?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=38803#action_38803 ] 

Chris Wilson commented on HHH-2007:
-----------------------------------

It would be pretty crazy for Hibernate to try to enforce this on the client end. What would it do if someone else inserted data into the database that Hibernate considers valid? Should Hibernate always perform a SELECT to look for duplicate values before allowing you to INSERT or UPDATE a record?

This should be enforced by the DBMS. I don't understand why you don't want it to be.

A UNIQUE constraint on the FK reference would normally be the correct solution in your case, as you don't want to share PKs. ANN-258 shows that this works for all databases except Derby. This is a limitation of Derby, not Hibernate.

Case closed IMHO.

> Hibernate doesn't support optional one-to-one associations
> ----------------------------------------------------------
>
>                 Key: HHH-2007
>                 URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-2007
>             Project: Hibernate Core
>          Issue Type: Bug
>          Components: core
>    Affects Versions: 3.2.0.cr2
>            Reporter: Andrei Iltchenko
>
> Hibernate doesn't properly support optional one-to-one associations, e.g. Person (0..1) -- (0..1) Passport. 
> 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. The first way enforces the mandatory property with the uniqueness constraint on the fk, the other by 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 
> 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 
> 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: 
>         <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: 
>  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. 
> Hibernate should be able to impose the single end property of such associations without relying on the underlying RDBMS engine and never allow them to degrade to many-to-ones.

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