Can I make an Appengine model property call a function?

Question

I'm implementing a frontpage with "hot" stories based on a certain ranking algorithm. However, I can't figure out how to pass Appengine's datastore my own sort function (like I can in python with sort(key=ranking_function)). I want something like this:

class Story(db.Model):
    user = db.ReferenceProperty(User)
    text = db.TextProperty()
    def ranking(self):
        # my ranking function, returns an int or something
        return 1
    ranking = property(ranking_function)

So that I can later call:

Story.all().order("ranking").limit(50)

Any idea how to do this using Appengine's datastore models?

Answer 1

I don't think this is possible with App Engine the way you describe it, but I think it is possible to achieve what you want. You want the datastore to run your ranking function against every element in the datastore, every time you do a query. That is not very scalable, as you could have millions of entities that you want to rank.

Instead, you should just have a integer property called rank, and set it every time you update an entity. Then you can use that property in your order clause.

Answer 2

There's no built in property that handles this, but there's a library, aetycoon, that implements DerivedProperty and other related properties that do what you want. Here's an article on how it works.