How to get data from 2 related tables in the same controller?

I'm trying to get data from 2 DB tables that are related. Departments and Functions: Each Department has multiple Functions.
I want to show all data in an accordion on the same page.
Until now, the code works like this: Declare the variable departments in the controller, and in view, in the foreach loop, I have called the Functions model in a variable.
Controller:

Code:

public function index(){
       //Data variable declarations
       $data['page_title'] = "Departments and Functions";
       $data['page_subtitle'] = "choose an occupation for the list of users assigned";

       //Data variable declaration - from models
       $data['departments'] = $this->Department->get_all('departments');

       $this->load->view('layouts/header');
       $this->load->view('layouts/title', $data);
       $this->load->view('layouts/aside');
       $this->load->view('departments/index', $data);
       $this->load->view('layouts/footer');
   }

Here I want to add the Function->get_by_department_id() method. How to get the ID dynamically?
The occupation model method:

Code:

public function get_by_fk($table = 'occupations', $fk = "department_id" , $fk_value=$department->department_id){
       $this->db->select('*');
       $this->db->from($table);
       $this->db->where($fk, $fk_value);
       $query = $this->db->get();

       return $result = $query->result();
   }