Using a custom property in the Google App Engine

Question

I am using the google engine / python application. And I created my own data type and property. But I have a problem with that. This is my definition.

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() 

and here is how I use it:

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

and I got the following error message:

 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'> 

I searched for a solution for a while and could not solve it. Thanks in advance if anyone knows how to fix this.

---

thanks for the answer. I realized that the main problem is that the user data type cannot get into the data store, so I need to wrap / deploy the user data type to / from the regular data type using get_value_for_datastore and make_value_from_datastore

this is my modified property definition:

 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) 

The key is to use get_value_for datastore to change the undirected MyDateTime object to a stored datetime object, and make_value_from_datastore vice versa.

Answer 1

The Property subclass must define the validate() method if its data type is not one of the defined data store data types that the default validator knows about. Here is an example validator method in this article .