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

appengine/findit/model/versioned_model.py

Index: appengine/findit/model/versioned_model.py
diff --git a/appengine/findit/model/versioned_model.py b/appengine/findit/model/versioned_model.py
new file mode 100644
index 0000000000000000000000000000000000000000..b59b90998a5deec697c4d3a6d7f449b4d1698464
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+++ b/appengine/findit/model/versioned_model.py
@@ -0,0 +1,101 @@
+# 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.
+"""
+
+import logging
+
+from google.appengine.api import datastore_errors
+from google.appengine.ext import ndb
+from google.appengine.runtime import apiproxy_errors
+
+
+class _GroupRoot(ndb.Model):
+  """Root entity of a group to support versioned children."""
+  # Key id of the most recent child entity in the datastore. It is monotonically
+  # increasing and is 0 if no child is present.
+  current = ndb.IntegerProperty(indexed=False, default=0)
+
+
+class VersionedModel(ndb.Model):
+  """A model that supports versioning.
+
+  Subclass will automatically be versioned, if use GetMostRecentVersion() to
+  read and use Save() to write.
+  """
+
+  @property
+  def version(self):
+    return self.key.integer_id()
+
+  @classmethod
+  def GetMostRecentVersion(cls):
+    """Returns the most recent version of the entity."""
+    assert not ndb.in_transaction()
+
+    root_key = cls._GetRootKey()
+    root = root_key.get()
+    if not root or not root.current:
+      return None
+
+    return ndb.Key(cls, root.current, parent=root_key).get()
+
+  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])
+      return True
+
+    def SetNewKey():
+      root.current += 1
+      self.key = ndb.Key(self.__class__, root.current, parent=root_key)
+
+    SetNewKey()
+    while True:
+      while self.key.get():
+        SetNewKey()
+
+      try:
+        if ndb.transaction(SaveData, retries=0):
+          return self.key
+      except (
+          datastore_errors.InternalError,
+          datastore_errors.Timeout,
+          datastore_errors.TransactionFailedError) as e:
+        # https://cloud.google.com/appengine/docs/python/datastore/transactions
+        # states the result is ambiguous, it could have succeeded.
+        logging.info('Transaction likely failed: %s', e)
+      except (
+          apiproxy_errors.CancelledError,
+          datastore_errors.BadRequestError,
+          RuntimeError) as e:
+        logging.info('Transaction failure: %s', e)
+      else:
+        SetNewKey()
+
+  @classmethod
+  def _GetRootModel(cls):
+    """Returns a root model that can be used for versioned entities."""
+    root_model_name = '%sRoot' % cls.__name__
+
+    class _RootModel(_GroupRoot):
+      @classmethod
+      def _get_kind(cls):
+        return root_model_name
+
+    return _RootModel
+
+  @classmethod
+  def _GetRootKey(cls):
+    return ndb.Key(cls._GetRootModel(), 1)