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kenmrph
 Post subject: list-index column not updated on delete
PostPosted: Thu Aug 14, 2008 4:10 pm 
Newbie

Joined: Thu Aug 14, 2008 3:47 pm
Posts: 4
Hibernate version: 3.2.6
MySQL version: 5.0.51a-community-nt

I'm having a problem with Hibernate not maintaining the list-index column in a many-to-many relationship. I've reproduced the problem in this simple test case:

I have a many-to-many relationship between Gang and Biker (I know: in real life, a biker would never belong to more than one gang; please suspend disbelief ;) The Gang side of the relationship uses a List of Biker objects, as the Bikers must be ordered within a given Gang. The biker side uses a Set of gangs. The latter side is marked inverse=true.

When a biker record is deleted using Session.delete(), the appropriate record is deleted from the join table, but the list-index column is not re-ordered, so that when a gang that formerly had that Biker as a member is accessed, a NullPointerException is thrown when iterating through that Gang's Biker List.

Here are my tables:

Code:
gang:
- id
- name

biker:
- id
- name

gang2biker:
- gang_id
- biker_id
- ordinal


And my mappings:

Code:
<class name="test.Gang"
   table="gang"
   lazy="true">
...
   <list name="bikers" lazy="true" table="gang2biker" cascade="save-update">
      <key column="gang_id"/>
      <list-index column="ordinal"/>
      <many-to-many column="biker_id" class="test.Biker"/>
   </list>            
</class>


<class name="test.Biker"
   table="biker"
   lazy="true">
...         
   <set name="gangs" lazy="true" table="gang2biker" cascade="save-update" inverse="true">
      <key column="biker_id"/>
      <many-to-many column="gang_id" class="test.Gang"/>
   </set>
</class>


Data in the join table prior to deleting might look like this:

Code:
gang_id        biker_id        ordinal
1                  1                  0
1                  2                  1
1                  3                  2


Now let's delete biker number 2:

Code:
session.delete(biker2);
tx.commit();


The data in the join table now looks like this:

Code:
gang_id        biker_id        ordinal
1                  1                  0
1                  3                  2


There is now a gap between the values in the "ordinal" (the list-index) column. The NPE is then thrown when iterating through List of Biker objects in a gang that formerly had that biker as a member:

Code:
for (Iterator i = g1.getBikers().iterator() ; i.hasNext() ; ) {
      Biker b = (Biker)i.next();
      System.out.println(" name: " + b.getName()); // <-- Throws NPE
}



I believe this is the result of the numerical gap in the list-index column.

I realize this issue has been discussed in the forums, but I've yet to come up w/ a workable solution. unfortunately, this problem was discovered in a system after it's been in use for some time, so a major re-architecture is problematic. I'm hoping for a a way to solve this at the hibernate configuration level. Is the only solution to iterate through all the gangs that have this biker as a member and delete it from each List?

Thanks for any help.
Ken
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kenmrph
 Post subject:
PostPosted: Thu Aug 14, 2008 7:10 pm 
Newbie

Joined: Thu Aug 14, 2008 3:47 pm
Posts: 4
OK... I think this is a case of me not fully grasping what hibernate can reasonably be expected to do. I guess what I need to do is not just delete the record, but remove it from any lists that have it as a member. e.g, instead of merely doing this:

Code:
session.delete(biker);


I need to do this:

Code:
for (Iterator i = biker.getGangs().iterator() ; i.hasNext() ; ) {
   Gang gang = (Gang)i.next();
   gang.getBikers().remove(biker);
}
session.delete(biker);


Have I seen the light?
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gatomx2
 Post subject:
PostPosted: Fri Aug 15, 2008 2:19 pm 
Newbie

Joined: Wed Aug 13, 2008 12:48 pm
Posts: 7
Location: Mérida, Yucatán, México
you'r right!!! ;)
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kenmrph
 Post subject: Problem implementing cleanup method in mapped object
PostPosted: Fri Aug 15, 2008 7:37 pm 
Newbie

Joined: Thu Aug 14, 2008 3:47 pm
Posts: 4
Got it! :) But I am now faced with a new wrinkle. It seems sensible to me to build the "cleanup" methods right into the mapped dataobjects. I.e., in my Biker object, I provide the method:

Code:
   public void cleanup() {
      for ( Iterator i = gangs.iterator() ; i.hasNext() ; ) {
         Gang g = (Gang)i.next();
         g.getBikers().remove(this);
      }
   }



I've made this method part of an interface "HouseKeeper", as my app may deal w/ mapped objects of different types, and I wish to be able to call it like so:

Code:
Object o = methodForLoadingObjectsOfVariousTypes(); // returns a Biker object in this case
HouseKeeper hk = (HouseKeeper)o;
hk.cleanup(); // first removes this biker from any gang's biker collections
sess.delete(o); // then delete this biker
tx.commit();


This produces the exception:

Code:
Exception in thread "main" org.hibernate.ObjectDeletedException: deleted object would be re-saved by cascade (remove deleted object from associations): [com.kqed.simplecms.test.list.Biker#33]
   at org.hibernate.impl.SessionImpl.forceFlush(SessionImpl.java:1014)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:165)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:94)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:70)
   at org.hibernate.impl.SessionImpl.fireSaveOrUpdate(SessionImpl.java:507)
   at org.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:499)
   at org.hibernate.engine.CascadingAction$5.cascade(CascadingAction.java:218)
   at org.hibernate.engine.Cascade.cascadeToOne(Cascade.java:268)
   at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:216)
   at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:169)
   at org.hibernate.engine.Cascade.cascadeCollectionElements(Cascade.java:296)
   at org.hibernate.engine.Cascade.cascadeCollection(Cascade.java:242)
   at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:219)
   at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:169)
   at org.hibernate.engine.Cascade.cascade(Cascade.java:130)
   at org.hibernate.event.def.AbstractFlushingEventListener.cascadeOnFlush(AbstractFlushingEventListener.java:131)
   at org.hibernate.event.def.AbstractFlushingEventListener.prepareEntityFlushes(AbstractFlushingEventListener.java:122)
   at org.hibernate.event.def.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:65)
   at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:26)
   at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1000)
   at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:338)
   at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:106)
   at com.kqed.simplecms.test.list.TestList.deleteById(TestList.java:126)
   at com.kqed.simplecms.test.list.TestList.main(TestList.java:24)


The funny thing is that this worked fine before I encapsulated the code into the biker object. That is, when I did the cleanup "inline", like so:

Code:
for (Iterator i = biker.getGangs().iterator() ; i.hasNext() ; ) {
   Gang gang = (Gang)i.next();
   gang.getBikers().remove(biker);
}
session.delete(biker);


It worked fine. Upon closer inspection with a debugger, I saw that in my calling code, my biker object reported itself to be:

Biker$$EnhancerByCGLIB$$2e8dd29a

So when I used this object when calling:

Code:
gang.getBikers().remove(biker);


follwed by:

Code:
session.delete(biker);


it works. However when I call:

Code:
biker.cleanup();


and step through that method internal to biker, I see that the this variable reports itself as a plain old Biker class (not the CGLIB generated class), so that when I call:

Code:
gang.getBikers().remove(this);


and later, after returning from the cleanup() method, call:

Code:
biker.cleanup();


... the results are bad, so that when I call tx.commit(), I get that exception.

I guess I'm expecting java to behave like java here, but the introduction of the CGLIB-generated objects is causing the PersistentSet.remove() method to not work as expected.

I would greatly appreciate any recommendations on how to handle this correctly!

Thanks!
Ken
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kenmrph
 Post subject:
PostPosted: Fri Aug 15, 2008 8:17 pm 
Newbie

Joined: Thu Aug 14, 2008 3:47 pm
Posts: 4
Once again I solved my own problem:

I needed to implement equals() and hashCode() in my dataobject.

(embarrassed)
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