Delete the child object via setting the parent's attribute to None.

Bug #1070335

We should not operate the foreigh key attribute of parent attribute
directly.

Change-Id: I98dc8eae78fdf67f0e7189e73afba77c6affcc0e

quantum/db/l3_db.py

@@ -186,7 +186,7 @@ def _update_router_gw_info(self, context, router_id, info):
         # figure out if we need to delete existing port
         if gw_port and gw_port['network_id'] != network_id:
             with context.session.begin(subtransactions=True):
-                router.update({'gw_port_id': None})
+                router.gw_port = None
                 context.session.add(router)
             self.delete_port(context.elevated(), gw_port['id'],
                              l3_port_check=False)

quantum/tests/unit/test_l3_plugin.py

@@ -630,6 +630,30 @@ def test_router_add_gateway(self):
                 gw_info = body['router']['external_gateway_info']
                 self.assertEquals(gw_info, None)
 
+    def test_router_update_gateway(self):
+        with self.router() as r:
+            with self.subnet() as s1:
+                with self.subnet() as s2:
+                    self._set_net_external(s1['subnet']['network_id'])
+                    self._add_external_gateway_to_router(
+                        r['router']['id'],
+                        s1['subnet']['network_id'])
+                    body = self._show('routers', r['router']['id'])
+                    net_id = (body['router']
+                              ['external_gateway_info']['network_id'])
+                    self.assertEquals(net_id, s1['subnet']['network_id'])
+                    self._set_net_external(s2['subnet']['network_id'])
+                    self._add_external_gateway_to_router(
+                        r['router']['id'],
+                        s2['subnet']['network_id'])
+                    body = self._show('routers', r['router']['id'])
+                    net_id = (body['router']
+                              ['external_gateway_info']['network_id'])
+                    self.assertEquals(net_id, s2['subnet']['network_id'])
+                    self._remove_external_gateway_from_router(
+                        r['router']['id'],
+                        s2['subnet']['network_id'])
+
     def test_router_add_gateway_invalid_network(self):
         with self.router() as r:
             self._add_external_gateway_to_router(