[hibernate-issues] [Hibernate-JIRA] Commented: (HBX-524) Reverse of one-to-one relationships

[Erik Lund Jensen (JIRA)]

    [ http://opensource.atlassian.com/projects/hibernate/browse/HBX-524?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_29988 ] 

Erik Lund Jensen commented on HBX-524:
--------------------------------------

Hi,

The mapping of one-to-one is a bit tricky to solve.
Firstly, Oracle specifies a couple of "laws":

Primary Key (PK) - Foreign Key (FK) LAW #1: If the foreign key column is part of the primary key (or part of an index) then the relationship between the tables will be one to many (1:M).              

Primary Key (PK) - Foreign Key (FK) LAW #2: If the foreign key column represents the entire primary key (or the entire index) then the relationship between the tables will be one to one  (1:1). 

See also http://www.sqlinfo.net/oracle/oracle_Create_foreign_key_contraints.php

Secondly, one-to-one may be mapped using the @PrimaryKeyJoinColumn, however, it is hard to decide the value of the attribute "optional", as Oracle schema does specify if the relation includes 1-zero. Further more, one-to-one relations may also be mapped without @PrimaryKeyJoinColumn.

I do not see, an general solution to this problem. One solution could be adding new methods to ReverseEngineeringStrategy to support one-to-one mapping and then let the application developer do the implementation details in the specific strategy.

/Erik Lund Jensen
Note:
Just an example of how one-to-one relation can be mapped:

@Entity 
public class Master 
{ 
@Id
private Long id; 


and the weak entity:

@OneToOne
@PrimaryKeyJoinColumn 
private Slave slave; 


> Reverse of one-to-one relationships
> -----------------------------------
>
>                 Key: HBX-524
>                 URL: http://opensource.atlassian.com/projects/hibernate/browse/HBX-524
>             Project: Hibernate Tools
>          Issue Type: Bug
>          Components: reverse-engineer
>    Affects Versions: 3.1beta2
>         Environment: HIbernate 3.1, Oracle 9i
>            Reporter: Andrea Cattani
>
> Hi,
> I've posted this issue to the forum and got this response from Max, Hibernate Team:
> "the reveng tools does not detect this as a one-to-one. it probably could, so add a request/patch to jira."
> The problem I've faced is the following:
> I have two tables, let's say
> - table A with column ID (PK) and other fields
> - table B with column ID (PK) and other fields
> table B has a foreign key constraint against table A, from column ID to column ID (one-to-one)
> When I reverese the tables with the HibernateTools I have such a resultant mapping for table B:
> <class name="B" table="B" schema="SCHEMA">
> <id name="id" type="string">
> <column name="ID" length="12" />
> <generator class="assigned" />
> </id>
> <[b]many-to-one name[/b]="a" class="A" update="false" insert="false" fetch="select">
> <column name="ID" length="12" not-null="true" unique="true" />
> </many-to-one>
> ....
> And this one for table A:
> <class name="A" table="A" schema="SCHEMA">
> <id name="id" type="string">
> <column name="ID" length="12" />
> <generator class="assigned"/>
> </id>
> <set name="b" inverse="true">
> <key>
> <column name="ID" length="12" not-null="true" unique="true" />
> </key>
> <[b]one-to-many[/b] class="B" />
> </set>
> </class>
> while I was expecting something like:
> [i]<one-to-one name="a" class="A" constrained="true"/>[/i]
> in table B, and the same (or nothing) in table A
> Thank you
> Andi

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