Hi,

I have a bidirecional many-to-many relation between classes A and B. A uses an association table AtoB and B uses an association table BtoA. I am using lists in both sides.

My ideia was to use AtoB and BtoA on the same table, creating a single four filed association table with the following fields:
1 - keyA
2 - keyB
3 - index AtoB
4 - index BtoA

The A side mapping would use fields 1, 2 and 3, and B side mapping would use fields 1, 2 and 4.

My question is: is it safe to do this? Or does it have to be two tables?

Thanks in advance,
Joao Rangel

Why do you use separate tables and colums ? This is not the appropriate mapping. Otherwise it's 2 different many to many links (not bidirectional)

Well, imagine the following situation: I have class user and class group. A user belongs to many groups, and a group may have many users. This is a many-to-many relation.

When I add a user to a group, I want to add the group to the user too. If I had one association table, instead of two, I may optimize the number of accesses and operations.

Is that right?

As I told you you do not need 2 tables or event 1 table and 4 columns.
1 table and 2 columns is enough. Actually the bidirectional many-to-many is done for that.

Have a look at the example in the reference guide (self many to many)



the key column of the first association is the manytomany column of the second one, and same for inverse.

I read the documentation many times, but only now i think i understand the meaning of inverse. In the documentation it says:

"Please note that Hibernate doesn't support bidirectional associations with list, map or array values (indexed collections) for the "many" end of a one-to-many association!"

But I am using lists. Is it possible to keep using lists, if I use diferent columns for the two indexes?

The documentation is wrong. It should say "one-to-many associations", not "associations".

actually the doco is correct, just badly worded. it says "one-to-many" at the end of the sentence.

I am getting a error because Hibernate doesn't find the "accessibleFrom" property in the source class. Shouldn't this property be mapped in the destination class?

If you have two classes, of course!

Emmanuel's example was for a recursive association.

Sorry I was too lazy to adapt the sample to your case ;-)

Yes, I have two classes and I am not making direct copy paste.

Here is may maps:

<class User>
...
<list name="objects" table="UserObject">
<key column="keyUser"/>
<index column="indexUser"/>
<many-to-many column="keyObject" class="Object"/>
</list>

<list name="users" table="UserObject" inverse="true">
<key column="keyObject"/>
<index column="indexObject"/>
<many-to-many column="keyUser" class="User"/>
</list>
</class>

<class Object>
</class>


In java classes, 'objects' belongs do the User class, and 'users' belongs to Object class. And this is the error:


SEVERE: Could not compile the mapping document
net.sf.hibernate.MappingException: Problem trying to set property type by reflection at net.sf.hibernate.mapping.Value.setTypeByReflection(Value.java:102)
at net.sf.hibernate.cfg.Binder.createProperty(Binder.java:956)
at net.sf.hibernate.cfg.Binder.propertiesFromXML(Binder.java:949)
at net.sf.hibernate.cfg.Binder.bindRootClass(Binder.java:312)
at net.sf.hibernate.cfg.Binder.bindRoot(Binder.java:1146)
at net.sf.hibernate.cfg.Configuration.add(Configuration.java:243)
at net.sf.hibernate.cfg.Configuration.addFile(Configuration.java:165)
at net.sf.hibernate.tool.hbm2ddl.SchemaExport.main(SchemaExport.java:282)
Caused by: net.sf.hibernate.PropertyNotFoundException: Could not find a getter for users in class User
at net.sf.hibernate.property.BasicPropertyAccessor.getGetter(BasicPropertyAccessor.java:186)
at net.sf.hibernate.util.ReflectHelper.reflectedPropertyType(ReflectHelper.java:57)
at net.sf.hibernate.mapping.Value.setTypeByReflection(Value.java:91)


I supposed that users should be in the destination class (Object). Why is Hibernate searching for it in the User side?

Cos you have it mapped on the user side, according to what you just showed!

change to:

<class User>
...
<list name="objects" table="UserObject">
<key column="keyUser"/>
<index column="indexUser"/>
<many-to-many column="keyObject" class="Object"/>
</list>
</class>


<class Object>
<list name="users" table="UserObject" inverse="true">
<key column="keyObject"/>
<index column="indexObject"/>
<many-to-many column="keyUser" class="User"/>
</list>
</class>

It's working... Thanks again for the Hibernate Team help.

I would like to make you one more question about this subject:
Is there any way to add a property to an association table?

Yes
section 7.2

A special case of a composite element is a composite element with a nested <many-to-one> element. A mapping like this allows you to map extra columns of a many-to-many association table to the composite element class. The following is a many-to-many association from Order to Item where purchaseDate, price and quantity are properties of the association: