python - 在 Google App Engine 中使用自定义属性

Question

我正在使用谷歌应用引擎/python。我已经创建了一个自定义数据类型和属性。但是我在使用它时遇到问题。这是我的定义

from datetime import datetime

class MyDateTime(datetime):
    def __str__(self):
        return self.strftime("%Y-%m-%d %H:%M")

class MyDateTimeProperty(db.Property):
    # specify datatype
    data_type = MyDateTime

    # For writing to datastore.
    def get_value_for_datastore(self, model_instance):
        return super(MyDateTimeProperty, self).get_value_for_datastore(model_instance)

    # For reading from datastore.
    def make_value_from_datastore(self, value):
        return super(MyDateTimeProperty, self).validate(value)

class MyClass(db.Model):
    date = MyDateTimeProperty()

这就是我使用它的方式:

obj = MyClass(date=MyDateTime(2011,1,1))
obj.put()

我收到以下错误消息:

Traceback (most recent call last):
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\admin\__init__.py", line 295, in post
    exec(compiled_code, globals())
  File "<string>", line 23, in <module>
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\db\__init__.py", line 1052, in put
    self._populate_internal_entity()
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\db\__init__.py", line 1020, in _populate_internal_entity
    self._entity = self._populate_entity(_entity_class=_entity_class)
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\db\__init__.py", line 1092, in _populate_entity
    self._to_entity(entity)
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\db\__init__.py", line 1005, in _to_entity
    self.__set_property(entity, prop.name, prop.get_value_for_datastore(self))
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\db\__init__.py", line 995, in __set_property
    entity[name] = datastore_value
  File "C:\Program Files\Google\google_appengine\google\appengine\api\datastore.py", line 881, in __setitem__
    datastore_types.ValidateProperty(name, value)
  File "C:\Program Files\Google\google_appengine\google\appengine\api\datastore_types.py", line 1477, in ValidateProperty
    'Unsupported type for property %s: %s' % (name, v.__class__))
BadValueError: Unsupported type for property date: <class '__main__.MyDateTime'>

找了好久没解决。如果有人知道如何修复它，请提前致谢。

---

感谢您的回答。我发现主要问题是自定义数据类型无法进入数据存储区，因此我需要使用 get_value_for_datastore 和 make_value_from_datastore

这是我修改后的属性定义:

class MyDateTimeProperty(db.Property):
    # specify datatype
    data_type = MyDateTime

    # For writing to datastore.
    def get_value_for_datastore(self, model_instance):
        date = super(MyDateTimeProperty, self).get_value_for_datastore(model_instance)
        return datetime(date.year, date.month, date.day, date.hour, date.minute)

    # For reading from datastore.
    def make_value_from_datastore(self, value):
        if value is None:
            return None
        return MyDateTime(value.year, value.month, value.day, value.hour, value.minute)

关键是，使用 get_value_for datastore 将不可存储的 MyDateTime 对象更改为可存储的 datetime 对象，而 make_value_from_datastore 反之亦然。

Answer 1

您的 Property 子类需要定义一个 validate() 方法，如果它的数据类型不是默认验证器知道的已定义数据存储数据类型之一。 this article 中有一个示例验证器方法.