如何读取 _pre_put_hook 中的旧属性值

我正在尝试实现 ndb 模型审核，以便对属性的所有更改都存储在每个模型实例中。这是我选择实现的 _pre_put_hook 的代码。

def _pre_put_hook(self):
    # save a history record for updates
    if not (self.key is None or self.key.id() is None):
        old_object = self.key.get(use_cache=True)
        for attr in dir(self):
            if not callable(getattr(self, attr)) and not attr.startswith("_"):
                if getattr(self, attr) != getattr(old_object, attr):
                    logging.debug('UPDATE: {0}'.format(attr))
                    logging.debug('OLD: {0} NEW: {1}'.format(getattr(old_object, attr), getattr(self, attr)))

问题是 old_object 始终填充与正在更新的 self（对象）相同的值。如何在实际执行 put() 之前访问旧对象的属性值 (_pre_put)？

EDIT:

随着时间的推移，我意识到我做了很多不需要做的工作（大量的 CPU/内存用于复制整个实体并在不需要时传递它们）。这是更新版本，它存储对原始 protobuf 的引用，并且仅在需要时才反序列化它

  __original = None    # a shadow-copy of this object so we can see what changed... lazily inflated
  _original_pb = None  # the original encoded Protobuf representation of this entity

  @property
  def _original(self):
    """
     Singleton to deserialize the protobuf into a new entity that looks like the original from database
    """
    if not self.__original and self._original_pb:
      self.__original = self.__class__._from_pb(self._original_pb)
    return self.__original

  @classmethod
  def _from_pb(cls, pb, set_key=True, ent=None, key=None):
    """
    save copy of original pb so we can track if anything changes between puts
    """
    entity = super(ChangesetMixin, cls)._from_pb(pb, set_key=set_key, ent=ent, key=key)
    if entity._original_pb is None and not entity._projection:
      # _from_pb will get called if we unpickle a new object (like when passing through deferred library)
      #   so if we are being materialized from pb and we don't have a key, then we don't have _original
      entity.__original = None
      entity._original_pb = pb
    return entity

当您第一次阅读该实体时，对其进行克隆：

在 Python 中复制 Google App Engine 数据存储中的实体，而无需在“编译”时知道属性名称 https://stackoverflow.com/questions/2687724/copy-an-entity-in-google-app-engine-datastore-in-python-without-knowing-property

并将其放在实体本身上，以便以后需要时可以引用。这样您就不必为了进行比较而读取第二个数据存储

我们重写两个不同的模型方法来实现这一点：

@classmethod
def _post_get_hook(cls, key, future):
    """
    clone this entity so we can track if anything changes between puts

    NOTE: this only gets called after a ndb.Key.get() ... NOT when loaded from a Query
      see _from_pb override below to understand the full picture

    also note: this gets called after EVERY key.get()... regardless if NDB had cached it already
      so that's why we're only doing the clone() if _original is not set...
    """
    entity = future.get_result()
    if entity is not None and entity._original is None:
        entity._original = clone(entity)

@classmethod
def _from_pb(cls, pb, set_key=True, ent=None, key=None):
    """
    clone this entity so we can track if anything changes between puts

    this is one way to know when an object loads from a datastore QUERY
      _post_get_hook only gets called on direct Key.get()
      none of the documented hooks are called after query results

    SEE: https://code.google.com/p/appengine-ndb-experiment/issues/detail?id=211
    """
    entity = super(BaseModel, cls)._from_pb(pb, set_key=set_key, ent=ent, key=key)
    if entity.key and entity._original is None:
        # _from_pb will get called if we unpickle a new object (like when passing through deferred library)
        #   so if we are being materialized from pb and we don't have a key, then we don't have _original
        entity._original = clone(entity)
    return entity