protected function _addIdField() {
        $params = array(
            'type' => 'numeric',
            'editable' => false,
            'visible' => false,
            //  next to being primary, an extra index key is also needed,
            //  to enable the flexibility to modify primary keys.
            'index' => true
        );

        $params['primary'] = !$this->_primaryKeyFieldIsPresent();

        $this['inputs'] = array('id' => $params) + $this['inputs'];
    }