数据库表示例：

CREATE TABLE IF NOT EXISTS `users` (
  `user_id` int(11) NOT NULL AUTO_INCREMENT,
  `first_name` varchar(100) NOT NULL,
  `last_name` varchar(100) NOT NULL,
  `username` varchar(100) NOT NULL,
  `password` text NOT NULL,
  `create_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (`user_id`),
  KEY `user_id` (`user_id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=latin1;

INSERT INTO `users` (`user_id`, `first_name`, `last_name`, `username`, `password`, `create_at`) VALUES
(1, 'ali', 'qorbani', 'aliqorbani', '63bed66f3d9dcd13440490d90738f816', '2018-07-28 08:34:01'),
(2, 'mohammad', 'ahmadi', 'mohammad', '316946a88ad51d75465c4c0b1e4a066a', '2018-07-28 08:34:01');

CREATE TABLE IF NOT EXISTS `user_groups` (
  `group_id` int(11) NOT NULL AUTO_INCREMENT,
  `group_name` varchar(100) NOT NULL,
  `create_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (`group_id`),
  KEY `group_id` (`group_id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=latin1;

INSERT INTO `user_groups` (`group_id`, `group_name`, `create_at`) VALUES
(1, 'superadmin', '2018-07-28 08:35:56'),
(2, 'admin', '2018-07-28 08:35:56'),
(3, 'moderator', '2018-07-28 08:36:12'),
(4, 'customer', '2018-07-28 08:36:12');

CREATE TABLE IF NOT EXISTS `user_roles` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `user_id` int(11) NOT NULL,
  `group_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `id` (`id`),
  KEY `user_id` (`user_id`),
  KEY `group_id` (`group_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

ALTER TABLE `user_roles`
  ADD CONSTRAINT `user_roles_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `users` (`user_id`) ON DELETE CASCADE ON UPDATE CASCADE,
  ADD CONSTRAINT `user_roles_ibfk_2` FOREIGN KEY (`group_id`) REFERENCES `user_groups` (`group_id`) ON DELETE CASCADE ON UPDATE CASCADE;
COMMIT;

你的模型代码是这样的。
//用户模型

public function add($data){
    $this->db->insert('users',$data);
    return true;
}
public function update($data,$id){
    $this->db->where('user_id',$id);
    $this->db->update('users',$data);
    return true;
}

public function add_user_roles($user_id,$roles = array()){
    $this->db->where('user_id',$user_id);
    $this->db->delete('user_roles');
    foreach ($roles as $role){
        $this->db->insert('user_roles',['user_id'=>$user_id,'group_id'=>$role]);
    }
    return true;
}

controlleredit方法的示例如下：

//Users controller
public function edit($user_id){
    if($this->form_validation->run() == FALSE) {
        $data_view['user'] = $this->User_model->get_user($user_id);
        $this->load->view('user_edit',$data_view);
    }else{
    $first_name = $this->input->post('first_name');
    $last_name = $this->input->post('last_name');
    $username = $this->input->post('username');
    $password = md5($this->input->post('password'));
    $user_roles = $this->input->post('roles');
    $data_user_update = array(
        'first_name'    =>  $first_name, 
        'last_name'     =>  $last_name, 
        'username'      =>  $username, 
        'password'      =>  $password
    );
    $this->User_model->add_user_roles$user_id,$user_roles);
    $this->User_model->update($data_user_update,$user_id);
    $this->session->set_flashdata('success','user updated correctly');
    redirect('users/edit/'.$user_id);
    }
}

这是我的（post，post\u tax，post\u tax\u relation）表截图。我想它能帮助你满足你的需要。记住 create , update 用户您必须更新关系表（我的意思是删除所有关系引用该用户，并在每次更新时创建新的关系）。

再也不要用这种方法了！创建两个不同的表（用户、用户组）。向用户表添加列 group_id 然后使用 join 函数获取主题有关详细信息，请参见此链接
如果你不明白，请把你的表格数据贴在这里，我会帮你：-）