tuple (5) to unlink all values in a many2many field doesn't work

According to the doc : http://doc.openerp.com/v6.0/developer/2_5_Objects_Fields_Methods/methods.html#osv.osv.osv.write
There is a command to unlink all targets of a m2m field :
(5) unlink all (like using (3,ID) for all linked records)

According to openobject-server.6.0/bin/osv/fields.py:626 :

elif act[0] == 5:
                cr.execute('update '+self._rel+' set '+self._id2+'=null where '+self._id2+'=%s', (id,))

This code leads to the following request :

update my_own_m2m_relation_table set target_id=null where target_id=43

This request is not correct, it leads to an UPDATE 0 each time. It should be :
update my_own_m2m_relation_table set target_id=null where source_id=43
So the correct code could be :

                cr.execute('update '+self._rel+' set '+self._id2+'=null where '+self._id1+'=%s', (id,))

But this is still false !
In the case of a many2many field, we have a separate table dedicated to linking the target values to the source field. So the correct request should be:

delete from my_own_m2m_relation_table where source_id=43

Thus the correct code should be something like:
                cr.execute('update '+self._rel+' set '+self._id2+'=null where '+self._id1+'=%s', (id,))