Re: [eclipselink-users] Problem with Master-Detail-Relationship

[christopher delahunt <christopher.delahunt@xxxxxxxxxx>]

Hello Michael,

This is expected because EclipseLink orders updates before it does 
deletes - allowing users to clean up references to avoid constraint 
issues that deletes might cause.  There is a flag you can set to have 
EclipseLink perform deletes first, set via the  
UnitOfWork.setShouldPerformDeletesFirst(boolean).  To get at the 
UnitOfWork in JPA, you can use

UnitOfWork uow = (UnitOfWork)( (JpaEntityManager)em.getDelegate()
).getActiveSession( em );
uow.shouldPerformDeletesFirst();

I'm not sure of any other way to set the flag at the moment.  As for 
paterns/anti-paterns, the application should not set the field to null 
if there is a not-null constraint on the field.  While the remove may 
happen immediately immediately after the null now, later on the 
application may do other work in between that could cause or require a 
flush, causing the same problem you are now in. 

Best Regards,
Chris

Michael Simons wrote:
> Hello,
>
> We've got a typical master-detail-relationship like:
> class A {
>   Set<B> details;
>   ...
>   removeDetail (B b) {
>      boolean removed = details.remove (b);
>      if (removed)
>        b.setMaster (null);
>      return removed;
>   }
> }
> class B {
>   A master;
>   ...
>   setMaster (A other) {
>     // if new master differs from old one we also remove
>     // this detail from the old master
>     if (master != null) {
> 	if (!master.equals (other)) {
> 		master.removeDetail (this);
> 		master = other;
> 		if (master != null)
> 			master.addDetail (this);
> 	}
>     } else if (other != null) {
> 	master = other;
> 	master.addDetail (this);
>     }
>   }
> }
>
> The problem occurs when a detail is removed persistently, because we first call
> A.removeDetail(detailToRemove) before entityManager.remove (detailToRemove);
>
> Doing so EclipseLink creates an update statement like
>   update detail_table set master_id = null where detail_id = ...;
> before the delete statement for the detail record.
> The update statement causes an exception, when the master_id column of the detail_table is NOT NULL.
>
> Is this behaviour intended?
> Is my code an anti-pattern?
>
> Any hints are appreciated.
>
> kind regards,
> Michael
>
>
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