Question

In some database technologies, for a attribute in a record, you can guarantee uniqueness of that attribute within the entire database. An example of this might be a email_address attribute in a User record. By setting email_address to unique, you guarantee that a particular email address can only appear in one record in the entire database.

Is there any way in google app engine to have unique properties for a given model? As a example, could I have a User(db.Model) entity with a email property that is guaranteed unique across the entire datastore?

I found this resource here, which might prove helpful.

Answer 1

I have implemented it in the following way by overriding the put method and using the key.name

class Program(db.Model):
    name = db.StringProperty("Title")
    slug = db.StringProperty("Slug")
    def put(self):
        if Program.get_by_key_name(self.slug):
            raise UniqueConstraintViolation("slug", self.slug)
        self._key_name = self.slug
        return db.Model.put(self)


class UniqueConstraintViolation(Exception):
    def __init__(self, scope, value):
        super(UniqueConstraintViolation, self).__init__("Value '%s' is not unique within scope '%s'." % (value, scope, ))

I save the slug as key.name and if you try to add another program it will check if the key name already exist. It’s probably not a nice way, im also a beginner at python / app engine.

This is good article about somebody using a helper model: http://squeeville.com/2009/01/30/add-a-unique-constraint-to-google-app-engine/

Edit: I saw you also provided that article lol.