[Findit] Re-org code.

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.
-"""
-
-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.
-
-  Subclasses will automatically be versioned. To create the first instance of a
-  versioned entity, use Create(key) with optional key to differentiate between
-  multiple unique entities of the same subclass. Use GetVersion() to read and
-  Save() to write.
-  """
-
-  @property
-  def _root_id(self):
-    return self.key.pairs()[0][1] if self.key else None
-
-  @property
-  def version_number(self):
-    # Ndb treats key.integer_id() of 0 as None, so default to 0.
-    return self.key.integer_id() or 0 if self.key else 0
-
-  @classmethod
-  def Create(cls, key=None):
-    """Creates an instance of cls that is to become the first version.
-
-    The calling function of Create() should be responsible first for checking
-    no previous version of the proposed entity already exists.
-
-    Args:
-      key: Any user-specified value that will serve as the id for the root
-        entity's key.
-
-    Returns:
-      An instance of cls meant to be the first version. Note for this instance
-        to be committed to the datastore Save() would need to be called on the
-        instance returned by this method.
-    """
-    return cls(key=ndb.Key(cls, 0, parent=cls._GetRootKey(key)))
-
-  @classmethod
-  def GetVersion(cls, key=None, version=None):
-    """Returns a version of the entity, the latest if version=None."""
-    assert not ndb.in_transaction()
-
-    root_key = cls._GetRootKey(key)
-    root = root_key.get()
-
-    if not root or not root.current:
-      return None
-
-    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, key=None):
-    root_entity = cls._GetRootKey(key).get()
-    if not root_entity:
-      return -1
-    return root_entity.current
-
-  def Save(self, retry_on_conflict=True):
-    """Saves the current entity, but as a new version.
-
-    Args:
-      retry_on_conflict (bool): Whether or not the next version number should
-        automatically be tried in case another transaction writes the entity
-        first with the same proposed new version number.
-
-    Returns:
-      The key of the newly written version, and a boolean whether or not this
-      call to Save() was responsible for creating it.
-    """
-    root_key = self._GetRootKey(self._root_id)
-    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():
-        if retry_on_conflict:
-          SetNewKey()
-        else:
-          # Another transaction had already written the proposed new version, so
-          # return that version's key and False indicating this call to Save()
-          # was not responsible for creating it.
-          return self.key, False
-
-      try:
-        if ndb.transaction(SaveData, retries=0):
-          return self.key, True
-      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:
-        if retry_on_conflict:
-          SetNewKey()
-        else:
-          # Another transaction had already written the proposed new version, so
-          # return that version's key and False indicating this call to Save()
-          # was not responsible for creating it.
-          return self.key, False
-
-  @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, key=None):
-    return ndb.Key(cls._GetRootModel(), key if key is not None else 1)