many-to-many: undesired deletes

alpha137 · **Joined:** Fri Feb 20, 2004 7:37 am **Posts:** 12

Sorry if this is a stupid question - I can't find the node in my head.

I have a many-to-many association between two classes A and B.
A sets inverse="false", B has inverse="true" and both have cascade="none".
I have the methods A.getBs() and B.getAs() to get the associated collections from both sides.

Now I'm working with an object A in different sessions. It's loaded in an EJB and transfered to client. I do not initialize the collection of Bs before transfering it. On the client A is changed, transfered back to the EJB and updated to the database with a simple session.update(A) using a different Hibernate session.
Now Hibernate deletes all my associations to the Bs (it does a "delete from <collection_table> where a_id=?").

I assume that Hibernate does the right thing - the collection of Bs in A isn't initialized.
But how can I update A and leave the associations untouched? I don't want to transfer the Bs down to the client and back (performance, network traffic, ...).

Many thanks for any help.

emmanuel · **Posted:** Wed Mar 31, 2004 10:14 am

do a specific addBsToA() and do the job in your server layer

alpha137 · **Joined:** Fri Feb 20, 2004 7:37 am **Posts:** 12

Hmm, I don't understand your point.
Adding B's to A works fine. I have such a function in my server layer where I do all the operations in one session (loading A, A.getBs().add(newB), ...).

My problem is when I just update A all existing associations to the B's are deleted.

I've tried to set both sides to inverse="true". Then the B's are not deleted.

But now I can't add or remove any associations. I've tried
A.getBs().add(newB)
newB.getAs().add(A)
session.flush()
=> no insert in the collection table.

emmanuel · **Posted:** Thu Apr 01, 2004 4:00 am

Do what I told you and in the server side
load A with it's Bs collection before adding new B, then Hibernate won't delete previous links

alpha137 · **Joined:** Fri Feb 20, 2004 7:37 am **Posts:** 12

Emmanual,
I thank you for your help and patience but we are not talking about the same thing.
I'll describe my problem again and in more detail:

On the server I load A:

Code:

A a = new A();
session.load(a, new Integer(<id>));

I don not load the B's here. No

Code:

// a.getBs();
// hibernate.initialize();

The A is transfered to the client. On the client I change a field - say:

Code:

a.setName("something");

The object is given back to the server for updating the changes:

Code:

session.saveOrUpdate(a);
session.flush();

Now two SQL-statements are executed:

Code:

update a set name=... where id=...
delete from <collection_table> where a_id=...

The second statement is my problem - it wipes all associations to Bs.
I think the reason is that the A is loaded in one session and updated in a different session.
At the moment I have the following solution - when updating the A:

Code:

A a2 = new A();
session.load(a2, a.getId());  //load the same A again in this session
a2.setData(a);   //copies all fields from a to a2
a2.update();
session.flush();

Now the delete-statement is not executed! It works but is this the only way to do it?

Thanks.

emmanuel · **Posted:** Thu Apr 01, 2004 12:31 pm

Are you doing something weird with Bs collection while detached ?
Is your getter/setter safe (no null set instead of the colelction set by hibernate...)

alpha137 · **Joined:** Fri Feb 20, 2004 7:37 am **Posts:** 12

Ok, I've left out one detail in my description above:
On the client side I have a higher-level business object where I copy the contents of A.
When updating the business object I create a new A object which is then transferred to the server. So the A loaded is different from the A updated and the collection of Bs is definitely null.

Hmm, are you sure that it would work when I would always use the same A? Even if it is transfered and marshalled through/over several proxies and stubs.

emmanuel · **Posted:** Fri Apr 02, 2004 1:45 pm

If you reassociate the objet graph to a new session, yes.

dannwebster · **Joined:** Wed Mar 24, 2004 7:40 pm **Posts:** 18

I have an identical situation and identical problem, so the answer to this question would answer my question as well. I think the question was answered, but I just want to make sure:

Quote:

alpha137:Hmm, are you sure that it would work when I would always use the same A? Even if it is transfered and marshalled through/over several proxies and stubs.

Quote:

emmanuel: If you reassociate the objet graph to a new session, yes.

The way I understand this, this means that if I use a different INSTANCE of class A to update the record corresponding to a1, the null collection of Bs will always delete all the A-to-B associations in the collection table.

By implication, this means that if I do this:

Code:

//session 1
A a1 = new A();
session1.load(a1, new Integer(<id>)); 

Code:

//go to client (web) tier where fields from a1 are dehydrated and cached

Code:

// session 2
A a2 = <rehydrate an A from cached fields>
session.saveOrUpdate(a2);
session.flush(); 

I will always do this:

Code:

update a set name=... where id=...
delete from <collection_table> where a_id=... 

So the best practical solution to this problem, which will keep my B collection intact is this:

Code:

A a2 = new A();
session.load(a2, a.getId());  //load the same A again in this session
a2.setData(a);   //copies all fields from a to a2
a2.update();
session.flush(); 

All this makes sense, but I guess what I am looking for is some kind of "please don't update this" placeholder to put in the collection of Bs. It seems like when I do a lazy, uninitialized load on the collection of Bs in a1, it must contain a Hibernate-specific implementation if an uninitialized collection that serves as this "please don't update this" placeholder.

Thus, it would be nice if we could get ahold of one of those to do something like this in session2:

Code:

// session 2
A a2 = <rehydrate an A from cached fields>
a2.setBs(<please don't update this placeholder>);//uninit'd collection?
session.saveOrUpdate(a);
session.flush(); 

Which would keep hibernate from deleting our B associations.

Is there anything like this possible?

dannwebster · **Joined:** Wed Mar 24, 2004 7:40 pm **Posts:** 18

Additionally, lets say A has an additional association to a group of Cs, this time one-to-many (instead of many-to-many). How come when I do a essentially this same pattern, my Cs do not get erased.

That is, when I have this:

Code:

<class name="A" >
        <id>...</id>
        <set name="Bs"
            lazy="true"
            table="collection_table"
            cascade="delete">
            <key column="A_ID"/>
            <many-to-many class="B" column="B_ID"/>
        </set>
       <set name="Cs"
            lazy="true"
            inverse="true"
            cascade="delete">
            <key column="A_ID"/>
            <one-to-many class="C"/>
        </set>
</class>

...and I do this:

Code:

//session 1
A a1 = new A();
session1.load(a1, new Integer(<id>));

Code:

//go to client (web) tier where fields from a1 are dehydrated and cached

Code:

// session 2
A a2 = <rehydrate an A from cached fields>
session.saveOrUpdate(a2);
session.flush(); 

...Hibernate does this:

Code:

update a set name=... where id=...
delete from <collection_table> where a_id=...

...but does not do this:

Code:

delete from <Cs table> where a_id =...

dannwebster · **Joined:** Wed Mar 24, 2004 7:40 pm **Posts:** 18

Is there additional information I can add to the above 2 questions to help anyone answer them? The two questions:

1) Is there a way to turn off the deletes for my Bs when updating A?
2) When I update my As, my Bs get deleted and not my Cs; why?

Thanks again.