如何在Codeigniter PHP中指定另一个数据库事件中减去数据库的值？

I want to subtract the Balance field of Reseller. Now Reseller can allot his balance to his users. So when a reseller adds balance to his users i want to deduct his balance by 5 EUROS(as the min and max balance he can allot is only 5). And There would be many reseller and many users. Each user is mapped by a unique of Reseller. So whenever a reseller add balance to his users only his balance should be deducted. It quite complicated, i think i must update the reseller balance field whenever the user is allotted balance. Please help me with my code.

The Reseller Controller :

public function edit ($id = NULL)
        {

            $usertype=$this->session->userdata('usertype');
            if($usertype ==="admin")
                {

                    // Fetch a user or set a new one
                    if ($id) {
                    $this->data['user'] = $this->reseller_m->get($id);
                    count($this->data['user']) || $this->data['errors'][] = 'User could not be found';
                }
            else 
                {
                    $this->data['user'] = $this->reseller_m->get_new();

                }

            // Set up the form
            $rules = $this->reseller_m->rules_admin;
            $id || $rules['password']['rules'] .= '|required';
            $this->form_validation->set_rules($rules);

                    // Process the form
            if ($this->form_validation->run() == TRUE) {

        $data = $this->reseller_m->array_from_post(array('sip_username','sip_password','key','allocation_block','name','email','password','phone','balance','user_num','address','country','country_code','created','modified','status'));


                $data['password'] = $this->reseller_m->hash($data['password']);

                $key=$this->reseller_m->save($data, $id);

                //here we get the last inserted record id in $last_id// 
                $last_id = $this->db->insert_id();

                //The logic to create key to uniquely identify a reseller.

                $values=array($this->input->post('name'),$this->input->post('country_code'),$last_id,$this->input->post('allocation_block'),$this->input->post('user_num'));

                $key=implode('-',$values);

                $this->db->where('id',$last_id);
                $this->db->update('reseller',array('key'=>$key));

                //Loop To Creates Users Dynamically as user_num value.

                $this->reseller_m->get($id);


                if($id === NULl)
                {
                    for($i=1; $i<=$data['user_num'];$i++)
                        {

                        $userdata=array('key'=>$key);

                        $this->db->insert('users',$userdata);

                        }
                }

                redirect('admin/reseller');

                }

                // Load the view
                $this->data['subview'] = 'admin/reseller/edit';
                $this->load->view('admin/_layout_main', $this->data);
                }
                else
                {

                    $this->load->view('permission');
                }


            }

The User Controller :

public function edit ($id = NULL)
    {

        $usertype=$this->session->userdata('usertype');
        if($usertype ==="admin")
            {

                // Fetch a user or set a new one
                if ($id)
                {
                    $this->data['user'] = $this->user_m->get($id);
                    count($this->data['user']) || $this->data['errors'][] = 'User could not be found';
                }
                else
                {
                    $this->data['user'] = $this->user_m->get_new();
                }

                // Set up the form
                $rules = $this->user_m->rules_admin;
                $id || $rules['password']['rules'] .= '|required';
                $this->form_validation->set_rules($rules);

                // Process the form
                if ($this->form_validation->run() == TRUE) {
                $data = $this->user_m->array_from_post(array('sip_id','sip_pass','name','email', 'password','phone','status','created','balance'));
                $data['password'] = $this->user_m->hash($data['password']);
                $this->user_m->save($data, $id);








                redirect('admin/user');
            }

        // Load the view
        $this->data['subview'] = 'admin/user/edit';
        $this->load->view('admin/_layout_main', $this->data);


            }


            else
                {
                    $this->load->view('permission');
                }

    }