如何读取_pre_put_hook中的旧属性值

Question

我正在尝试实施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总是填充有更新的self（对象）的相同值。如何在实际生成put（）之前访问旧对象的属性值（_pre_put）？

Answer 1

修改

随着时间的推移，我意识到我正在做一些不需要做的工作（很多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，它存储了对原始protobuf的引用，只有在需要时才反序列化

  __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

---

首次阅读时复制实体：

Copy an entity in Google App Engine datastore in Python without knowing property names at 'compile' time

并将其放在实体本身上，以便以后可以在需要时引用它。这样，您不必为了进行比较而执行第二次数据存储读取

我们覆盖两种不同的模型方法来实现这一目标：

@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