[Findit] Add a model to provide versioning and also a verioned config model.

appengine/findit/model/versioned_config.py

Index: appengine/findit/model/versioned_config.py
diff --git a/appengine/findit/model/versioned_config.py b/appengine/findit/model/versioned_config.py
new file mode 100644
index 0000000000000000000000000000000000000000..c0e3447573a77d9b9accf72b8f4f0640e809da7c
--- /dev/null
+++ b/appengine/findit/model/versioned_config.py
@@ -0,0 +1,50 @@
+# 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.
+
+"""Versioned singleton entity with the global configuration."""
+
+import logging
+
+from google.appengine.api import users
+from google.appengine.ext import ndb
+
+from model.versioned_model import VersionedModel
+
+
+class VersionedConfig(VersionedModel):
+  """Singleton entity with the global configuration of the service.
+
+  All changes are stored in the revision log.
+  """
+
+  # When this revision of configuration was created.
+  updated_ts = ndb.DateTimeProperty(indexed=False, auto_now_add=True)
+
+  # Who created this revision of configuration.
+  updated_by = ndb.StringProperty(indexed=False)
+
+  @classmethod
+  def Get(cls):
+    """Returns the current up-to-date version of the config entity."""
+    return cls.GetMostRecentVersion() or cls()
+
+  def Update(self, **kwargs):
+    """Applies |kwargs| dict to the entity and stores the entity if changed."""
+    if not users.is_current_user_admin():
+      raise Exception('Only admin could update config.')
+
+    dirty = False
+    for k, v in kwargs.iteritems():
+      assert k in self._properties, k
+      if getattr(self, k) != v:
+        setattr(self, k, v)
+        dirty = True
+
+    if dirty:
+      user_name = users.get_current_user().email().split('@')[0]
+      self.updated_by = user_name
+      self.Save()
+      logging.info('Config %s was updated by %s', self.__class__, user_name)
+
+    return dirty