Hibernate - how to delete bidirectional many-to-many association
- by slomir
Problem:
I have many-to-many association between two entities A and B.
I set A entity as an owner of their relationship(inverse=true is on A's collection in b.hbm.xml).
When i delete an A entity, corresponding records in join table are deleted.
When i delete an B entity, corresponding records in join table are not deleted (integrity violation exception).
--
Let's consider some very simple example:
class A{
Set<B> bset=new HashSet<B>();
//...
}
class B{
Set<A> aset=new HashSet<A>();
//...
}
File a.hbm.xml [m-to-m mappings only]:
<set name="bset" table="AB">
<key name="a_id"/>
<many-to-many column="b_id" class="B"/>
</set>
File b.hbm.xml [m-to-m mappings only]:
<set name="aset" table="AB" inverse="true">
<key name="b_id"/>
<many-to-many column="a_id" class="A"/>
</set>
Database relations:
A(id,...)
B(id,...)
AB(a_id,b_id)
Suppose that we have some records in AB joint table. For example:
AB = {(1,1),(1,2)}
where AB= { (a_id , b_id) | ... ... }
--
Situation 1 - works probably because A is owner of AB relationship:
A a=aDao.read(1); //read A entity with id=1
aDao.delete(a); //delete 'a' entity and both relations with B-entities
Situation 2 - doesn't work:
B b=bDao.read(1); //read B entity with id=1
bDao.delete(b); //foreign key integrity violation
On the one hand, this is somehow logical to me, because the A entity is responsible for his relation with B.
But, on the other hand, it is not logical or at least it is not orm-like solution that I have to explicitly delete all records in join table where concrete B entity appears, and then to delete the B entity, as I show in situation 3:
Situation 3 - works, but it is not 'elegant':
B b=bDao.read(1);
Set<A> aset=b.getA(); //get set with A entities
Iterator i=aset.iterator();
//while removes 'b' from all related A entities
//while breaks relationships on A-side of relation (A is owner)
while(i.hasNext()){
A a=i.next();
a.bset.remove(b); //remove entity 'b' from related 'a' entity
aDao.update(a); //key point!!! this line breaks relation in database
}
bDao.delete(b); //'b' is deleted because there is no related A-entities
--
So, my question: is there any more convenient way to delete no-owner entity (B in my example) in bidirectional many-to-many association and all of his many-to-many relations from joint table?
