Why does hibernate/jpa set @OrderColumn field to null for a removed element in a mapped list before deleting it? -

i'd map tree structure of "chapters". every chapter has reference parent, , ordered list (by "position") of subchapters. jpa 2.0, hibernate 3.5, entity "chapter" looks follows:

@entity public class chapter {      @id     @generatedvalue     private long id;      @column(name="position", nullable=false)     private int position;      @manytoone     @joincolumn(name="parentchapter_id", updatable=false, insertable=false)     private chapter parentchapter;      @onetomany(cascade=cascadetype.all, orphanremoval=true)     @ordercolumn(name="position")     @joincolumn(name="parentchapter_id")     private list<chapter> subchapters = new arraylist<chapter>();      public list<chapter> getsubchapters() {             return subchapters;     } } 

the problem is, if 1 of elements of subchapters removed

// entitymanager em chapter parent = em.find(chapter.class, 1); subchapters = parent.getsubchapters(); subchapters.remove(1); entitytransaction tx = em.gettransaction(); tx.begin(); em.persist(parent); tx.commit(); 

then hibernate tries execute statement

update     chapter  set     parentchapter_id=null,     position=null      parentchapter_id=?      , id=? 

which fails because of not null constraint of position. if @ordercolumn(name="position") removed, hibernate doesn't update position (and therefore works) , removes (sub)chapter afterwards. causes hibernate first update future orphan , then remove it?

the problem conceptual one, specially position property :-) it's property of list itself, not list-element (chapter). so, differently:

@entity  public class chapter {   @id @generatedvalue   private long id;    @manytoone   private chapter parent;    @onetomany(mappedby="parent")   @ordercolumn(name="position")   private list<chapter> children; } 

note have 2 structs here: chapter , list. point list state must persistent, it's not related struct chapter. so, deserves table (and that's hibernate try if enable "generate ddl").

you can see example test suite: https://github.com/hibernate/hibernate-core/tree/master/hibernate-core/src/test/java/org/hibernate/test/collection/list/

you'll see not using annotations, can translate :-)