[Hibernate-issues] [Hibernate-JIRA] Commented: (HB-174) Allow multi-column discriminators and other discriminator improvements

[<leg...@at...>]

The following comment has been added to this issue:

     Author: Tom Sedge
    Created: Thu, 10 Jul 2003 8:43 AM
       Body:
I forgot to say that the single-column discriminator is presumed by the persister/loader architecture via getDiscriminatorColumnName().  This is one of the reasons it is so difficult to extend this at the moment.

What about a completely separate and extensible discriminator sub-system - would something like this be feasible?  (I lack sufficient in-depth knowledge of Hibernate's sourcecode to judge this myself).
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Here is an overview of the issue:
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        Key: HB-174
    Summary: Allow multi-column discriminators and other discriminator improvements
       Type: Improvement

     Status: Unassigned
   Priority: Major

    Project: Hibernate2
   Versions:
             2.0.1

   Assignee: 
   Reporter: Tom Sedge

    Created: Thu, 10 Jul 2003 8:40 AM
    Updated: Thu, 10 Jul 2003 8:40 AM
Environment: N/A

Description:
The extensible persister system in Hibernate is very nice and useful and custom discriminator types can be created via a custom Hibernate
type and new persister.  However, the structure of the discriminator is not wholly under the persister's control.

Recently I've come across a number of situations where mapping legacy data models requires one of:

1. A joined-subclass discriminator where the discriminator in the parent table may be in more than one column (e.g. Column A not null implies Class A, Column B not null implies Class B).

2. A joined-subclass discriminator where the discriminator is not the primary key of the parent table.

3. A joined-subclass discriminator where force="true" can be applied.

4. Nesting a table-per-class-hierachy within joined-subclass (e.g. Table A contains parent class A, Table B joins to that table for class B and also contains Class C via a discriminator in Table B) & vice-versa.

5. Generating a discriminator from multiple columns via a general expression.

As far as I can see (2) is not possible as there is no syntax support for this in the XML document (i.e. specify column in parent in <key> directive).  (1) follows from (2) - requires specifying the column.  (3) doesn't seem to be availale for joined-subclasses but is important since only some of the data in table may be needed.  (4) looks completely ruled out by the DTD.  (5) looks difficult, though a custom persister might be able to fudge it.

What are the chances of Hibernate supporting any of the above?  What is the level of difficulty involved in each one (I may be able to help).

For the record, these seem to me to be reasonable requirements and would greatly ease working with difficult legacy data model - which in my experience are more common than greenfield sites.


 


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