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POUsing Custom Property in Google App Engine
 

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  1. POUsing Custom Property in Google App Engine
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    <p>I'm using google app engine/python. And I have created a custom datatype and property. But I have problem using it. This is my definition</p> <pre><code>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() </code></pre> <p>and this is how I use it:</p> <pre><code>obj = MyClass(date=MyDateTime(2011,1,1)) obj.put() </code></pre> <p>and I got the following error message:</p> <pre><code>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 "&lt;string&gt;", line 23, in &lt;module&gt; 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: &lt;class '__main__.MyDateTime'&gt; </code></pre> <p>I've searched for solution for a while and could not solve it. Thanks in advance if anyone knows how to fix it.</p> <hr> <p>thanks for the answer. I figured out that the prime problem is that custom data type can't go into datastore, so I need to wrap/unwrap the custom datatype to/from usual datatype using <code>get_value_for_datastore</code> and <code>make_value_from_datastore</code></p> <p>this is my modified property definition:</p> <pre><code>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) </code></pre> <p>The key is, using <code>get_value_for datastore</code> to change an unstorable MyDateTime object to storable datetime object, and <code>make_value_from_datastore</code> vice versa.</p>
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    1. USLasse Vågsæther Karlsen
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      1. USАндрей Татаринов
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    1. COIf this is real code rather than demonstrative, you might consider handling date formatting in a template filter rather than a custom property.
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      1. POUsing Custom Property in Google App Engine
      1. USDrew Sears
    2. COI agree with Drew - this is a lot of hoops to jump through just to get custom behaviour on `str`.
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      1. POUsing Custom Property in Google App Engine
      1. USNick Johnson
 

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