objects: Makes sure Instance._save methods are called

This patch makes sure that _save hooks are called for Instance object's
related objects even when they are set to None (if they are at all
nullable)

We will need this for numa_topology, as we need to be able to update it
on save, and make sure it's gone if the new one is None.

Change-Id: Ied9e760b40446661a6269e41df6091aa16fbfc82

nova/objects/instance.py

@@ -410,10 +410,12 @@ class Instance(base.NovaPersistentObject, base.NovaObject):
         delattr(self, base.get_attrname('id'))
 
     def _save_info_cache(self, context):
-        self.info_cache.save(context)
+        if self.info_cache:
+            self.info_cache.save(context)
 
     def _save_security_groups(self, context):
-        for secgroup in self.security_groups:
+        security_groups = self.security_groups or []
+        for secgroup in security_groups:
             secgroup.save(context)
         self.security_groups.obj_reset_changes()
 
@@ -479,9 +481,10 @@ class Instance(base.NovaPersistentObject, base.NovaObject):
 
         updates = {}
         changes = self.obj_what_changed()
+
         for field in self.fields:
             if (self.obj_attr_is_set(field) and
-                    isinstance(self[field], base.NovaObject)):
+                    isinstance(self.fields[field], fields.ObjectField)):
                 try:
                     getattr(self, '_save_%s' % field)(context)
                 except AttributeError: