CakePHP如何处理带有/不带'id'字段的HABTM表？

public $hasAndBelongsToMany = array(
    'Group' => array(
        'className' => 'Group',
        'joinTable' => 'users_groups',
        'foreignKey' => 'user_id',
        'associationForeignKey' => 'group_id',
        'unique' => true,
        'conditions' => '',
        'fields' => '',
        'order' => '',
        'limit' => '',
        'offset' => '',
        'finderQuery' => '',
        'deleteQuery' => '',
        'insertQuery' => ''
    )
);

public $hasAndBelongsToMany = array(
    'User' => array(
        'className' => 'User',
        'joinTable' => 'users_groups',
        'foreignKey' => 'group_id',
        'associationForeignKey' => 'user_id',
        'unique' => true,
        'conditions' => '',
        'fields' => '',
        'order' => '',
        'limit' => '',
        'offset' => '',
        'finderQuery' => '',
        'deleteQuery' => '',
        'insertQuery' => ''
    )
);

public function add() {
    if ($this->request->is('post')) {
        $this->User->create();
        if ($this->User->save($this->request->data)) {
            $this->Session->setFlash(__('The user has been saved.'));
            return $this->redirect(array('action' => 'index'));
        } else {
            $this->Session->setFlash(__('The user could not be saved. Please, try again.'));
        }
    }
    $groups = $this->User->Group->find('list');
    $this->set(compact('groups'));
}

$user = $this->User->find('first', array(
    'conditions' => array('User.id' => $userId),
    'contain' => array('Group')
));