[Findit] A huge refactoring and some bug fixing.

appengine/findit/model/versioned_model.py

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Provides a model to support versioned entities in datastore.
 
 Idea: use a root model entity to keep track of the most recent version of a
 versioned entity, and make the versioned entities and the root model entity in
 the same entity group so that they could be read and written in a transaction.
 """
@@ skipping 36 matching lines @@
     if version is None:
       version = root.current
     elif version < 1:
       #  Return None for versions < 1, which causes exceptions in ndb.Key()
       return None
 
     return ndb.Key(cls, version, parent=root_key).get()
 
   @classmethod
   def GetLatestVersionNumber(cls):
-    return cls._GetRootKey().get().current
+    root_entity = cls._GetRootKey().get()
+    if not root_entity:
+      return -1
+    return root_entity.current
 
   def Save(self):
     """Saves the current entity, but as a new version."""
     root_key = self._GetRootKey()
     root = root_key.get() or self._GetRootModel()(key=root_key)
 
     def SaveData():
       if self.key.get():
         return False  # The entity exists, should retry.
       ndb.put_multi([self, root])
@@ skipping 35 matching lines @@
 
       @classmethod
       def _get_kind(cls):
         return root_model_name
 
     return _RootModel
 
   @classmethod
   def _GetRootKey(cls):
     return ndb.Key(cls._GetRootModel(), 1)