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From Andrus Adamchik <and...@objectstyle.org>
Subject Re: Deletes before Inserts?
Date Tue, 11 May 2010 06:06:30 GMT
>
> Is there a reason why INSERTs occur before DELETEs?

Yes, the current operation ordering is designed for presumably the  
most common scenario. But it is definitely not working for all cases.  
Specifically INSERT goes in before DELETE, as deleted objects may have  
FK references from other objects replaced by inserted object. So  
DELETE can only be done after INSERT and UPDATE.

> Say the "key" column needs to be unique and you delete
> the record with key="timeout" and insert another new record with  
> key="timeout".

Some edge cases of this scenario (e.g. delete/insert of the same join  
record of a many-to-many) are handled by Cayenne as an UPDATE. But  
others where PK is different for two records but some other UNIQUE  
column is same can't be handled as UPDATE.

I'd go as far as saying that without deferred constraint checking at  
the DB level it is not even possible to always satisfy an arbitrary  
set of DB constraints just by reordering operations. So a user  
solution to that would be doing multiple commits (I know this sucks)...

On Cayenne end we can investigate the same multi-commit approach, but  
done in the same internal transaction, so that (a) it is transparent  
to the user and (b) an atomic rollback is possible. Not sure if that  
approach would satisfy constraint checkers on all DB's though.

Andrus



On May 10, 2010, at 6:50 PM, Michael Gentry wrote:

> Is there a reason why INSERTs occur before DELETEs?

> It looks like
> this is the bit of code that orders it (at least in 3.0):
>
> DataDomainFlushAction.java / preprocess
>
>        insertBucket.appendQueries(queries);
>        flattenedBucket.appendInserts(queries);
>        updateBucket.appendQueries(queries);
>        flattenedBucket.appendDeletes(queries);
>        deleteBucket.appendQueries(queries);
>
>
> There seems to be an issue if you have a DB constraint (unique column,
> for example).  Say the "key" column needs to be unique and you delete
> the record with key="timeout" and insert another new record with
> key="timeout".  The commit will fail because the insert occurs before
> the delete.
>
> Thanks,
>
> mrg
>


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