def dirty_vals(model):
        """ Get the models dirty values in a friendly SQL format

        This will be a string of comma separated field
        names in a format for psycopg to substitute with
        parameterized inputs.

        An example, if the `rid` & `created` fields are
        dirty then they'll be converted into:

            '%(rid)s, %(created)s'

        :return: str or None
        """

        vals = []

        for field in model.dirty_fields:
            vals.append('%({0})s'.format(field))

        vals = ', '.join(vals)

        return vals or None