Adapted from the Mongo docs::

                session.query(Places).filter(Places.loc.within_radius(1, 2, 50)

def within_radius(self, x, y, radius):
        """
            Adapted from the Mongo docs::

                session.query(Places).filter(Places.loc.within_radius(1, 2, 50)
        """
        return QueryExpression({
            self : {'$within' : {
                    '$center' : [[x, y], radius],
                }}
            })