Hibernate is create an extra select query to map collections on an entity merge operation

Solution Verified - Updated -

Environment

  • JBoss Enterprise Application Platform (EAP) 5.1
  • Hibernate 3.3

Issue

  • We are  seeing one extra select query to Map Collections after a merge operation on the owning entity.  The map was not modified so there is nothing to change, and if session.update instead of merge is used, there is no extra select query.

  • During the merge operation the map collections become dirty, due to this the additional select is triggered:

    TestManagerNew [Java Application]   
  at test.TestManagerNew at localhost:40513   
  at Thread [main] (Suspended (breakpoint at line 191 in PersistentMap))   
  at PersistentMap.put(Object, Object) line: 191   
  at MapType.replaceElements(Object, Object, Object, Map, SessionImplementor) line: 98   
  at MapType(CollectionType).replace(Object, Object, SessionImplementor, Object, Map) line: 574   
  at TypeFactory.replace(Object[], Object[], Type[], SessionImplementor, Object, Map) line: 505   
  at DefaultMergeEventListener.copyValues(EntityPersister, Object, Object, SessionImplementor, Map) line: 496   
  at DefaultMergeEventListener.entityIsDetached(MergeEvent, Map) line: 424   
  at DefaultMergeEventListener.onMerge(MergeEvent, Map) line: 234   
  at DefaultMergeEventListener.onMerge(MergeEvent) line: 84   
  at SessionImpl.fireMerge(MergeEvent) line: 706   
  at SessionImpl.merge(String, Object) line: 690   
  at SessionImpl.merge(Object) line: 694

Resolution

In order to merge the map per JPA definition Hibernate has to know its current "persistent" state.  However, as the owner and collection are detached, the only known "persistent state" is the database.  This is true for merge in general.

update() is a different case, with different semantics.  It assumes that things reattached are dirty and will always result in updates.

This is all logical and consistent if you step back and think about the fact that update() does not return a new instance while merge() does.

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