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From "Karan Malhi" <Karan.Ma...@learnquest.com>
Subject RE: question
Date Fri, 29 Jun 2007 15:20:03 GMT
Thanks Craig,
 
Some more follow up questions.
 "Anyway, the implementation is required to actually assign an id only 
at the point in time when you either flush the instance or ask for 
the object id via getObjectId. Once assigned, the id won't change."
 
so if I do pm.makePersistent(obj);
Object objectid = pm.getObjectId(obj);
I am assuming JDO will assign this object an ID. lets say it is 10
What happens if I rollback the transaction?
Also, what happens if before I commit, some other tx uses 10 and inserts a row
When I commit, will i get an exception? This would be somewhat unexpected I guess, so I think
I did not understand it properly
 
"The assignment of the object id depends to a large extent on the 
user's metadata for the class. Several strategies can be used."
 
Where exactly in the metadata do I specify this?
 
Thanks for the help
Regards

Karan Singh


________________________________

From: Craig L Russell [mailto:Craig.Russell@Sun.COM]
Sent: Thu 6/28/2007 8:34 PM
To: Apache JDO project; JDO Expert Group
Subject: Re: question



Hi Karan,

In JDO, an object id is like Schroedinger's cat [1]. You don't know 
what it is until you ask for it, or the implementation needs it. This 
is because in many cases, you need to round trip to the database to 
create the id and it's more efficient if you can defer the database 
trip until the transaction commits.

Anyway, the implementation is required to actually assign an id only 
at the point in time when you either flush the instance or ask for 
the object id via getObjectId. Once assigned, the id won't change.

Of course, there's nothing to stop the implementation from eagerly 
assigning the id during the makePersistent call.

The assignment of the object id depends to a large extent on the 
user's metadata for the class. Several strategies can be used.

Craig

[1] http://en.wikipedia.org/wiki/Schrödinger's_cat
On Jun 28, 2007, at 1:50 PM, Karan Malhi wrote:

> Does a persistent-new object have an objectID. If yes, then what is
> the value stored in that? How does JDO determine what value to store
> in it?
>
> --
> Karan Malhi

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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