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From Craig L Russell <Craig.Russ...@Sun.COM>
Subject Re: Issue 145: Mapping Map with embedded PersistenceCapable
Date Tue, 22 Nov 2005 04:01:33 GMT
Hi Abe,

I think I now understand your point, which I missed earlier. You  
don't see a need for a primary key to be defined on a join table.

The issue is whether a compliant JDO implementation can require that  
a join table must have a primary key if it is used to implement a  
multi-valued field (collection or map).

Apparently, JPOX requires this, and there is nothing in the spec  
currently that addresses the issue at all. Unless we change the spec  
to disallow the JPOX implementation, it seems that it is de facto a  
portability issue.

What are the implications of not having a primary key in the join  
table? The constraint that the Map keys (or Set elements) must be  
unique must be enforced by the application (the JDO implementation).  
This implies that the entire Map or Set must be instantiated in  
memory in order to prevent duplicates. [I don't see how to implement  
a lazy loading strategy for such Maps or Sets.]

What else?

Craig

On Nov 21, 2005, at 6:42 PM, Abe White wrote:
>> If owner_id is primary key and nothing else, we will not be able  
>> to store more
>> than one instance of Dog for each owner. Thus, we need a new  
>> column to be part
>> of the primary key.
>>
>
> Or the table could have *no primary key at all*, which is what I'm  
> arguing should be perfectly portable.
>
>> Right. They have no persistent identity. Therefore, if you want to  
>> use an embedded PersistenceCapable as a key, you need to identify  
>> one of the embedded fields as a key for the purpose of the map.
>> The alternative is to simply disallow using an embedded  
>> PersistenceCapable as a key in a map if  the class is defined as  
>> datastore identity. But this seems pretty arbitrary.
>
> The only requirements for using objects in a Map in Java are that  
> they implement hashCode and equals properly.  How do you go from  
> that contract to "needs a primary key in the database map table"?

Craig Russell
Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
408 276-5638 mailto:Craig.Russell@sun.com
P.S. A good JDO? O, Gasp!


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