Set a relation n-n

function film_management()
{
    $crud = new grocery_CRUD();
 
    $crud->set_table('film');
    $crud->set_relation_n_n('actors', 'film_actor', 'actor', 'film_id', 'actor_id', 'fullname','priority');
    $crud->set_relation_n_n('category', 'film_category', 'category', 'film_id', 'category_id', 'name');
 
    $crud->unset_columns('description','special_features','last_update');
    $crud->fields('title', 'description', 'actors' ,  'category' ,'release_year', 'rental_duration', 'rental_rate', 'length', 'replacement_cost', 'rating', 'special_features');
 
    $output = $crud->render();
 
    $this->_example_output($output);
}