diff --git a/openerp/osv/fields.py b/openerp/osv/fields.py
index f55e4b9c027..58a7dfec715 100644
--- a/openerp/osv/fields.py
+++ b/openerp/osv/fields.py
@@ -525,7 +525,13 @@ class one2many(_column):
                 # Must use write() to recompute parent_store structure if needed
                 obj.write(cr, user, [act[1]], {self._fields_id:id}, context=context or {})
             elif act[0] == 5:
-                cr.execute('update '+_table+' set '+self._fields_id+'=null where '+self._fields_id+'=%s', (id,))
+                reverse_rel = obj._all_columns.get(self._fields_id)
+                assert reverse_rel, 'Trying to unlink the content of a o2m but the pointed object does not have a m2o'
+                # if the pointed object has on delete cascade, just delete it
+                if reverse_rel.column.ondelete == "cascade":
+                    obj.unlink(cr, user, obj.search(cr, user, [(self._fields_id,'=',id)], context), context)
+                else:
+                    cr.execute('update '+_table+' set '+self._fields_id+'=null where '+self._fields_id+'=%s', (id,))
             elif act[0] == 6:
                 # Must use write() to recompute parent_store structure if needed
                 obj.write(cr, user, act[2], {self._fields_id:id}, context=context or {})