Using Property Builtin with GAE Datastore's Model

Question

I want to make attributes of GAE Model properties. The reason is for cases like to turn the value into uppercase before storing it. For a plain Python class, I would do something like:

Foo(db.Model):
    def get_attr(self):
       return self.something
    def set_attr(self, value):
       self.something = value.upper() if value != None else None
    attr = property(get_attr, set_attr)

However, GAE Datastore have their own concept of Property class, I looked into the documentation and it seems that I could override get_value_for_datastore(model_instance) to achieve my goal. Nevertheless, I don't know what model_instance is and how to extract the corresponding field from it.

Is overriding GAE Property classes the right way to provides getter/setter-like functionality? If so, how to do it?

Added:

One potential issue of overriding get_value_for_datastore that I think of is it might not get called before the object was put into datastore. Hence getting the attribute before storing the object would yield an incorrect value.

Answer 1

Subclassing GAE's Property class is especially helpful if you want more than one "field" with similar behavior, in one or more models. Don't worry, get_value_for_datastore and make_value_from_datastore are going to get called, on any store and fetch respectively -- so if you need to do anything fancy (including but not limited to uppercasing a string, which isn't actually all that fancy;-), overriding these methods in your subclass is just fine.

Edit: let's see some example code (net of imports and main):

class MyStringProperty(db.StringProperty):
    def get_value_for_datastore(self, model_instance):
        vv = db.StringProperty.get_value_for_datastore(self, model_instance)
        return vv.upper()

class MyModel(db.Model):
    foo = MyStringProperty()

class MainHandler(webapp.RequestHandler):

    def get(self):
        my = MyModel(foo='Hello World')
        k = my.put()
        mm = MyModel.get(k)
        s = mm.foo
        self.response.out.write('The secret word is: %r' % s)

This shows you the string's been uppercased in the datastore -- but if you change the get call to a simple mm = my you'll see the in-memory instance wasn't affected.

But, a db.Property instance itself is a descriptor -- wrapping it into a built-in property (a completely different descriptor) will not work well with the datastore (for example, you can't write GQL queries based on field names that aren't really instances of db.Property but instances of property -- those fields are not in the datastore!).

So if you want to work with both the datastore and for instances of Model that have never actually been to the datastore and back, you'll have to choose two names for what's logically "the same" field -- one is the name of the attribute you'll use on in-memory model instances, and that one can be a built-in property; the other one is the name of the attribute that ends up in the datastore, and that one needs to be an instance of a db.Property subclass and it's this second name that you'll need to use in queries. Of course the methods underlying the first name need to read and write the second name, but you can't just "hide" the latter because that's the name that's going to be in the datastore, and so that's the name that will make sense to queries!

Answer 2

What you want is a DerivedProperty. The procedure for writing one is outlined in that post - it's similar to what Alex describes, but by overriding get instead of get_value_for_datastore, you avoid issues with needing to write to the datastore to update it. My aetycoon library has it and other useful properties included.