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From "Craig Russell (JIRA)" <j...@apache.org>
Subject [jira] Commented: (JDO-204) Need attribute on field element for type
Date Wed, 16 Nov 2005 22:11:29 GMT
    [ http://issues.apache.org/jira/browse/JDO-204?page=comments#action_12357824 ] 

Craig Russell commented on JDO-204:

I understand that some implementations have the ability to store completely different classes
in a persistent field. But there is currently no standard way to describe how this is done.
For example, in JDO1 TCK we had a field of Object and stored String and Long in the field
in different instances. And if you serialize the contents of the field, there is no issue.
But now we allow the user to specify the mapping for embedded and referenced storage. And
there is no standard way for the user to tell the implementation how to store these. You would
at least need some discriminator column to store the class of the instance.

Without having a standard way to describe the storage of heterogeneous instances, I'm reluctant
to go too far in standardizing the classes that can be stored in the field. But this is different
from the proposal, which is to allow overriding of the compiled field type at deployment time,
for example to specify that even though the object model says that there is an Object stored
in a field, the only class that can be stored there is a SimpleClass. 

That said, I don't object to using the field-type to name a list of classes that can be stored,
and using the <extension> metadata to describe in more detail how the storage is to
be done.

> Need attribute on field element for type
> ----------------------------------------
>          Key: JDO-204
>          URL: http://issues.apache.org/jira/browse/JDO-204
>      Project: JDO
>         Type: Bug
>   Components: api20
>     Reporter: Michelle Caisse
>     Assignee: Craig Russell

> Need to supply instantiated type information for Object & Interface fields.

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