Hi,

I am having some difficulty with a bidirectional many-to-many relationship.
I have read up on bidirectional relationships - but I am having some problem - most likely something I am doing wrong - something I misunderstand.

I have two entities that have a many-to-many relationship. Ala:

<class name="com.toot.toot.User" table="UsersSandbox" mutable="false">
...snip...
<set name="categories" lazy="false" cascade="none" table="UserCategory">
<key column="user_id"/>
<many-to-many class="com.toot.toot.Category" column="category_id"/>
</set>
</class>

<class name="com.toot.toot.Category" table="Category">
...snip...
<set name="subscribers" lazy="false" cascade="none" table="UserCategory" >
<key column="category_id"/>
<many-to-many class="com.toot.toot.User" column="user_id" />
</set>
</class>

At time=0, We have a User U and a Category C - they have been loaded by a session.find query - and they have no relationships. I want to create a bidirectional association between them.

So, I do the following:
user.getCategories().add(category);

Now, if I understand correctly, for bidirectional relationships, I also need to do the following:
category.getSubscribers().add(user);

However, this results in Hibernate trying to insert two relationships in the Join Table.... which violates our constraints.

So, I guess my question is: "What am I doing wrong?". What am I not understanding?

If I just do the 1st one, then what gets persisted to the database is correct, however, until I clear the cached values (session.clear()), and reload the entities, my category C still shows no subscribers.... :-(

Thanks.
-Nick

Using JDK1.4.2, Hib2.1.2

I don't think both your classes should have a many to many- only the "parent " object should have the many-many. the category should have a 1-m- not m-m.

Code should just need to add to the one set. Potentially if inverse=true, parent will need setting on child.

Good luck

James

??

Not sure I understand why. Can you point me to some doco?

Cheers,
Nick

Think I was wrong- looks like you can have a set on both ends. Anyway, the example I would use is author-work example:
http://www.hibernate.org/hib_docs/refer ... xamples-s1

Notice there is a table called "author_work" that is not in the object model.

Unfortuneately, seems that there is no code w/ this example, so I'm not sure you would add to both the sets...

James

Author/Work is the appropriate example. The Hibernate solution to this problem seems to be that, in a bi-directional many-to-many relationship, one side has to be "less equal" than the other. One side has more responsibility for managing the relation.

Add inverse = "true" to the Category side:

and then

will not result in the duplicate relation being added to the UserCategory table.

This is because inverse="true" says that Category does not have responsibility for creating the SQL that manages the relation. User does have that responsibility becuase inverse="false" is the default.

More about inverse is at http://www.hibernate.org/155.html

Will

Thanks v.much Will.

I think I get it now.

-Nick

I think that the point about Hibernate (took me a little bit for the penny to drop) is that it is "Idiomatic Persistance".

Adding to only one set wouldnt make sense in Java for a bidirectional relationship. So if Hibernate were to do some magic behind the scenes then it would be "intrusive". Ie your code wouldnt work without Hibernate (like with CMP/CMR).

So, you *want* to add to both collections - but you want Hibernate to only do the one persistance-related update/insert.

The key seems to be inverse="true"

-Nick