[hibernate-issues] [Hibernate-JIRA] Commented: (HHH-1233) Allow mixing <union-subclass>and <subclass>

[Steve Ebersole (JIRA)]

    [ http://opensource.atlassian.com/projects/hibernate/browse/HHH-1233?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_27125 ] 

Steve Ebersole commented on HHH-1233:
-------------------------------------

And it makes the most sense to move this discussion over to the dev list.  But don't expect us to tell you how to implement the patch.  That would require us to investigate it to the degree that we might as well implement it ourselves.  We will certainly help you out with specific questions you might run into...

> Allow mixing <union-subclass>and <subclass>
> -------------------------------------------
>
>                 Key: HHH-1233
>                 URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-1233
>             Project: Hibernate3
>          Issue Type: Improvement
>          Components: core
>         Environment: N/A
>            Reporter: Gunther Schadow
>
> BACKGROUND:
> Hibernate allows mixing <subclass> and <joined-subclass> inheritance mappings but apparently not <subclass> and <union-subclass> mappings, however, this seems like the most useful combination. Example:
> USE CASE:
> We may have a hierarchy with very unevenly distributed mass, i.e., some subclasses have millions of objects, others have only a half-dozen. There may be a lot of different subclasses of the smaller sort.
> Union-subclass seems ideal to work with a dozen different humongous tables, as they can be placed on different tablespaces and managed differently. However, for the 200 or so subclasses whith only a dozen objects each, it would be easier to map these into a single table per sub-hierarchy.
> REQUESTED FEATURE:
> Therefore it seems fair to suggest that <subclass> mappings should be supported underneath <union-subclass> mappings. To allow that the <discriminator> element would have to be handled inside a <union-subclass> element and same for the discriminator-value attribute. Perhaps one could limit the complexity by requiring that the outer mapping must be union-subclass and the inner can be subclass but not vica versa?
> WORKAROUNDS:
> As the documentation mentions one could just create several independent <class> hierarchies, thus effectively emulating the first level of union-subclass specializations. But that would not allow specifying polymorphic associations.
> ESTIMATED EFFORT:
> It could be difficult to implement support for this. But would like to know whether this is something that is somewhere on the radar screen for the future.

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