在 Google App Engine 中使用自定义属性
我正在使用谷歌的应用引擎和Python。我创建了一个自定义的数据类型和属性，但在使用时遇到了问题。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()
这是我使用它的方式：
obj = MyClass(date=MyDateTime(2011,1,1))
obj.put()
然后我收到了以下错误信息：
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'>
我搜索了解决方案一段时间，但还是没能解决。如果有人知道怎么修复这个问题，先谢谢你。
谢谢你的回答。我发现主要问题是自定义数据类型不能直接存入数据存储，所以我需要用 get_value_for_datastore 和 make_value_from_datastore 来把自定义数据类型转换成普通的数据类型。
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)
关键在于，使用 get_value_for_datastore 将不能存储的 MyDateTime 对象转换为可以存储的 datetime 对象，而 make_value_from_datastore 则是反向操作。