import wsgiref.handlers
from google.appengine.ext import db, webapp

class MyModel(db.Model):
    first = db.StringProperty()
    last = db.StringProperty()

class UniqueConstraint(db.Model):
    @classmethod
    def check(cls, model, **values):
        # Create a pseudo-key for use as an entity group.
        parent = db.Key.from_path(model.kind(), 'unique-values')

        # Build a list of key names to test.
        key_names = []
        for key in values:
            key_names.append('%s:%s' % (key, values[key]))

        def txn():
            result = cls.get_by_key_name(key_names, parent)
            for test in result:
                if test: return False
            for key_name in key_names:
                uc = cls(key_name=key_name, parent=parent)
                uc.put()
            return True

        return db.run_in_transaction(txn)

class TestHandler(webapp.RequestHandler):
    def get(self):
        def write(msg):
            self.response.out.write(msg + '\n')
        
        def create(first, last):
            if UniqueConstraint.check(MyModel, first=first, last=last):
                write('Creating %s %s!' % (first, last))
                mm = MyModel(first=first, last=last)
                mm.put()
                return True
            else:
                write('Darnit, "%s" or "%s" is taken!' % (first, last))
                return False

        write('<pre>')
        write('Clearing data!')
        for mm in MyModel.all():
            mm.delete()
        for uc in UniqueConstraint.all():
            uc.delete()
        write('Done!')
        write('---')

        for i in xrange(4):
            if not create('Bob', 'Doe'):
                if not create('Bob', 'Foo'):
                    if not create('Bar', 'Foo'):
                        if not create('Jane', 'Boo'):
                            write('We\'re all out of luck!')
            write('---')

        write('<strong>Data in the datastore:</strong>')

        for mm in MyModel.all():
            write('MyModel: %s %s' % (mm.first, mm.last))

        for uc in UniqueConstraint.all():
            write('UniqueConstraint: %s &rarr; %s &rarr; %s' % (
                uc.key().parent().kind(), uc.key().parent().name(),
                uc.key().name()))

        write('</pre>')

def main():
    application = webapp.WSGIApplication([
        ('/test', TestHandler),
    ])
    wsgiref.handlers.CGIHandler().run(application)

if __name__ == '__main__':
    main()