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From Craig L Russell <Craig.Russ...@Sun.COM>
Subject Re: Collection (or Map) relationships and null
Date Tue, 12 Dec 2006 23:09:18 GMT
Hi Dain,

On Dec 12, 2006, at 2:33 PM, Dain Sundstrom wrote:

> On Dec 12, 2006, at 1:45 PM, Craig L Russell wrote:
>> There's nothing that I can find in the JPA specification about  
>> preserving null vs. empty Collections and Maps. There's no good  
>> (practical) way to implement null values for these types in  
>> relational databases, which is the target of the specification.
>> The only way I know to do it is to redundantly add a column to the  
>> table whose mapped class contains the Collection or Map field that  
>> knows whether an empty Collection or Map should be presented to  
>> the user as null. This was deemed by the JPA experts to be non- 
>> trivial and non-demanded by users.
>> I am curious why this issue came up while trying to implement CMP  
>> 2 via JPA.
> CMP2 requires that collection valued fields always be non-null  
> (i.e., you always get an empty collection).  I wanted to know what  
> guarantees JPA provided in terms of the field being not null.   
> Specifically, if the field is set to null when an empty collection  
> is loaded, I would have to compensate for that in my framework  
> code.  Since, JPA does the intelligent thing and inserts an empty  
> collection, my job is easier.

Yup. All you really need to do is to instantiate an empty collection  
in the constructor and you're home free.

> -dain

Craig Russell
Architect, Sun Java Enterprise System
408 276-5638
P.S. A good JDO? O, Gasp!

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