'Subcommand of alter_column that updates nulls to default value (overrideable)'

def _update_nulls_to_default(self, params, field):

    default = field.get_db_prep_save(field.get_default(), connection=self._get_connection())
    self.execute(('UPDATE    %(table_name)s    SET    %(column)s=%%s    WHERE    %(column)s    IS    NULL' % params), [default])
