Hi,
I,ve got these two classes that have a many-to-many relationship :
Usuario<----->Modulo
so there is a UserModule table that contains data.

Usuario class has a Set of Modulos.
I can successfully add modules to any user, but I can

Yes it

Can you post your mapping for the Usuario class, especially the portion defining the relation to Modulo, and the Modulo class mapping.

Does Modulo have a reference to Usuario?

What does your hashCode() method look like?

Ok, Usuario:

<hibernate-mapping>
<class
name="org.siia.aplicaciones.siiadmin.Usuario"
table="USUARIOS_GIS"
proxy="org.siia.aplicaciones.siiadmin.Usuario"
>

<id
name="id"
column="C_USUARIO"
type="long"
unsaved-value="0"
>
<generator class="sequence">
<param name="sequence">usuario_seq</param>
</generator>
</id>

<property
name="administrador"
type="boolean"
update="true"
insert="true"
column="L_ADMISTRADOR"
not-null="true"
/>

<set
name="modulos"
table="USUARIOS_MODULOS"
lazy="true"
inverse="false"
cascade="all"
sort="unsorted"
>

<key
column="USUARIO_ID"
/>

<many-to-many
class="org.siia.aplicaciones.siiadmin.Modulo"
column="MODULO_ID"
/>

</set>
.
.
.

And Modulo doesn

Remember that the proxy is a different object! Accessing its instance variables directly in equals() is just not going to work. Use the getter.

THAT WAS IT!!!!

I was using something like this in equals:

...
if ( m.id == id )
return true;


So it was wrong, the correct thing is:

if ( m.getId() == getId() )
return true;

Now, it works
Thank you very much you all!!!!