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Re: [eclipselink-users] Pre-existing managed entity instances in the persistence context: pre-existing on disk? Or...?
No, Andrei I believe was referring to EclipseLink searching the cache for the entity, and if its not there, building a new instance. If its not in the cache, EclipseLink by default will check the database to see if the object is existing and use that data to build the managed instance. So you will not get a duplicate insert when using merge with EclipseLink with the code you have provided. 

Best Regards,
Chris



On 26/07/2013 12:52 AM, Laird Nelson wrote:

On Thu, Jul 25, 2013 at 1:02 PM, Andrei Ilitchev
<andrei.ilitchev@xxxxxxxxxx <mailto:andrei.ilitchev@xxxxxxxxxx>> wrote:

    Merge searches persistence context for entity with the same id as
    the detached being merged to put merge result into it (say, it has
    been read before through the same entity manager and em.clear hasn't
    been called).
    If none found then merged result placed into a new entity.


OK, so I *will* get a primary key duplication error *by spec*?

Recall the code is:

    final SomeEntity x = new SomeEntity();
    x.setID(6); // set the @Id -- the persistent identity

    //
    // Assumption: x is now "detached"; it is not "new"
    // Please correct this assumption if it is false.
    //

    em.clear(); // empty persistence context

    // 1

    final SomeEntity xPrime = em.merge(x); // 2

    em.flush(); // 3 -- Duplicate primary key error? If not, why not?


So I will get a primary key duplication error at point 3 *by spec*, yes?
  Because at point 2 a new entity with the same persistent identity was
introduced into the persistence context?

Many, many people are laboring under the delusion that everything will
be just fine.  They think that at point 3 the spec requires the JPA
provider to conduct a search for the right row on disk and to UPDATE it
if one is found.  You are saying that an INSERT might very well
happen--and fail, in this case.  Right?

Best,
Laird

--
http://about.me/lairdnelson


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