HI,

I was reading "Hibernate in Action" about many-to-many bidirectional relationship. It says that this kind of relationship has to have one end which will actualy do the updates to the database when calling saveOrUpdate.

I am wondering why can't Hibernate update the database form both sides?
Or can it?

You can update the database from both ends. Set the cascade="save-update" on both the one and many side of the mapping.

Example:

On the one side, the cascade option can be set for the set of objects:
<set name="foo" inverse="true" cascade="save-update">
...
</set>

On the many side, set the property of the parent to also cascade:
<many-to-one name="bar" class="com.bar" cascade="save-update">
...
</many-to-one>

This way, you can create both foo and bar objects, and cascade save them.

I do this when I have an entity set like this:

Parent Type1 -> Child <- Parent Type2.

I'll take a known parent, add a new child with a new parent type 2, and save the whole thing by calling save on just the type 1 object.

Hope this helps!

My question was about many-to-many relaton, not meny-to-one. With many-to-one it works fine, but many-to-many is the problem.

This issue applies if you want the Java Collection updates to be equal to the database sync state. So, in the case of a many-to-many in Java you set both sides of the collection thus mapping has to have one side set as inverse=true so you dont get two updates (which will conflict) to the database. If you are only setting one side of the relationship in Java then no need to have the inverse true on any side - but be carefull as the Java collection will not match the database. In many cases, your throwing the collection away after the unit-of-work so its fine.

So, if I don't want synchronization with database on every add or remove from my collection, but on the end of the transaction it's Ok to remove inverse="true" from collection mapping on both sides?

Yes - Experiment a bit and see.