model proxy: Throw custom exception when model not cached

Currently, when attempting to access fields on a model which is not
cached, the esception thrown complains that NoneType doesn't have the
attribute. This message does not say much.

This change makes the exception include the proxy object which failed
cache-lookup. It also adds a log message stating the lookup failed.

Change-Id: I6e77eeefa44307f92b7c40be7349a07ce5b67407

dragonflow/common/exceptions.py

@@ -52,6 +52,10 @@ class DBKeyNotFound(DragonflowException):
     message = _('DB Key not found, key=%(key)s')
 
 
+class ReferencedObjectNotFound(DragonflowException):
+    message = _('Referenced object not found. proxy=%(proxy)s')
+
+
 class CommandError(DragonflowException):
     message = _("Non-existent fields are specified: %(non_existent_fields)s")
 

dragonflow/db/model_proxy.py

@@ -12,10 +12,16 @@
 import copy
 import six
 
+from oslo_log import log
+
 from dragonflow._i18n import _
+from dragonflow.common import exceptions
 from dragonflow.db import db_store
 
 
+LOG = log.getLogger(__name__)
+
+
 class _ModelProxyBase(object):
     '''Base for proxy objects
 
@@ -74,8 +80,12 @@ class _ModelProxyBase(object):
         return not self == other
 
     def __getattr__(self, name):
-        if name != '_obj':
-            return getattr(self.get_object(), name)
+        if name == '_obj':
+            return
+        obj = self.get_object()
+        if obj is None:
+            raise exceptions.ReferencedObjectNotFound(proxy=self)
+        return getattr(obj, name)
 
     def __copy__(self):
         return self.__class__(self._id)

dragonflow/tests/unit/test_model_proxy.py

@@ -10,10 +10,12 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 import copy
+import testtools
 
 from jsonmodels import fields
 import mock
 
+from dragonflow.common import exceptions
 from dragonflow.db import db_store
 import dragonflow.db.field_types as df_fields
 import dragonflow.db.model_framework as mf
@@ -179,3 +181,9 @@ class TestObjectProxy(tests_base.BaseTestCase):
         self.assertEqual(model_test_ref, model_test_ref_deepcopy)
         self.assertNotEqual(id(model_test_ref), id(model_test_ref_copy))
         self.assertNotEqual(id(model_test_ref), id(model_test_ref_deepcopy))
+
+    def test_non_existing_reference(self):
+        reffing_model = RefferingModel(id='2', model_test='1')
+        self.db_store.get_one.return_value = None
+        with testtools.ExpectedException(exceptions.ReferencedObjectNotFound):
+            topic = reffing_model.model_test.topic  # noqa