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From "Fay Wang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (OPENJPA-782) Support for collections of embeddables and basic types
Date Wed, 11 Mar 2009 23:42:50 GMT

    [ https://issues.apache.org/jira/browse/OPENJPA-782?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12681095#action_12681095
] 

Fay Wang commented on OPENJPA-782:
----------------------------------

Spec 2.5 Embeddable Classes (p 34):

"Since instances of embeddable classes themselves have no persistent identity, the relationship
from the referenced entity is to the entity that contains the embeddable instance(s) and not
to the embeddable itself."

Spec 10.1.26 ManyToOne Annotation (p. 310):

"The ManyToOne annotation may be used within an embeddable class to specify a relationship
from the embeddable class to an entity class."

Spec 10.1.35 OneToOne Annotation (p.323):

"The OneToOne annotation may be used within an embeddable class to specify a relationship
from the embeddable class to an entity class. "

Case (1):

EntityA
======
private Collection<EmbedA> embedAs;

EmbedA:
========
@ManyToOne
EntityB entityB;

The values for EntityA, EmbedA, EntityB can be:

EntityA1 - EmbedA1 - EntityB1
                   EmbedA2 - EntityB1
                   EmbedA3 - EntityB2
                   EmbedA4 - EntityB2

The implied relation from EntityA to EntityB is therefore OneToMany. A bi-directional relation
between EntityA and EntityB (spec p.40) suggests that the relation from EntityB to EntityA
is ManyToOne.

EntityB:
=======
@ManyToOne
private EntityA entityA;


Case(2):

EntityA
======
private Collection<EmbedA> embedAs;

EmbedA:
========
@OneToOne
EntityB entityB;


The values for EntityA, EmbedA, EntityB can be:

EntityA1 - EmbedA1 - EntityB1
                   EmbedA2 - EntityB2
                   EmbedA3 - EntityB3
                   EmbedA4 - EntityB4

The implied relation from EntityA to EntityB is therefore OneToMany. A bi-directional relation
between EntityA and EntityB (spec p.40) suggests that the relation from EntityB to EntityA
is ManyToOne.

EntityB:
=======
@ManyToOne
private EntityA entityA;


In both cases, EntityA should be the owning side (as it contains EmbedA). In order to specify
bi-directional relation, the mappedBy annotation should be specified in EntityB, However,
EntityB has ManyToOne relation, which disallows "mappedBy" to be specified. Therefore, IMO,
it is not possible to specify bi-directional relation when a collection embeddables is involved.


> Support for collections of embeddables and basic types
> ------------------------------------------------------
>
>                 Key: OPENJPA-782
>                 URL: https://issues.apache.org/jira/browse/OPENJPA-782
>             Project: OpenJPA
>          Issue Type: Sub-task
>            Reporter: Fay Wang
>            Assignee: Fay Wang
>             Fix For: 2.0.0
>
>         Attachments: openjpa-782-testcase.patch, openjpa-782.patch
>
>
> Regarding jpa 2.0 embeddable, suppose Entity A contains a collection of embeddables.
> The collection of embeddable is stored in a separate table. @CollectionTable can be used
to specify the table name, etc. If the @CollectionTable is not used, a default table name
is used (see spec, p 272 ). 
> The embeddable may contain the relation to EntityB in the following scenarios:
> (1) If in the embeddable, there is:
>       @ManyToOne
>        private EnittyB entityB; 
>    
>        then the relation from EntityA to EntityB is OneToMany, 
>        and the relation from EntityB to EntityA is ManyToOne (Spec 2.5):
>        @ManyToOne
>        private EntityA entityA;
> (2) If in the embeddable, there is:
>       @OneToOne
>       private EntityB; 
>       then the relation from EntityA to EntityB is OneToMany,
>       and the relation from EntityB to EntityA is ManyToOne (Spec 2.5):
>       @ManyToOne
>       private EntityA entityA;
> (3) If in the embeddable, there is:
>       @OneToOne(mappedBy = "entityB")
>        private EntityB
>        The relation from EntityA to EnityB is OneToMany,
>        and the relation from EntityB to EntityA is ManyToOne:
>        @ManyToOne
>        private EntityA entityA;       
> The relation to EntityB will be stored in the third table (the CollectionTable) so that
it can be associated with the embeddable. However, we can not use "mappedBy" in the ManyToOne
relation in EntityB. How to have bi-directional relation between EntityA and EntityB in these
situations? Is it possible to have a bi-directional relation between these two entities when
embeddable is involved as described above?
> In scenario (3), an explicit "mappedBy" is specified in the embeddable, so that the bi-directional
relation between EntityA and EntityB will be stored in the EntityB table, but then how can
we associate EntityB with the embeddable when EntityA is retrieved back as embeddable itself
does not have identity?

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