Need refactoring advice

Hello, all.

Here is my model superclass (used by several child models):

PHP Code:

<?php

class MY_Model extends CI_Model
{

    protected $table = '';

    protected function switch_db($webinar_id)
    {
        if (! $this->db->db_select('webinar_' . $webinar_id)) die('Вебинар не существует');
    }

    public function get($webinar_id, $id = 0, $filter = [])
    {
        $this->switch_db($webinar_id);
        $where_clause = ($id == 0 ? [] : ['id' => $id]) + $filter;
        $query = $this->db->get_where($this->table, $where_clause);
        return ($id == 0 ? $query->result_array() : $query->row_array());
    }

    public function get_only_status($webinar_id, $id = 0, $filter = [])
    {
        $this->switch_db($webinar_id);
        $where_clause = ($id == 0 ? [] : ['id' => $id]) + $filter;
        $query = $this->db->select('id, status')->get_where($this->table, $where_clause);
        return ($id == 0 ? $query->result_array() : $query->row_array());
    }

    public function add($webinar_id, $data)
    {
        $this->switch_db($webinar_id);
        $data['status'] = 0;
        $data['date'] = date('Y-m-d H:i:s');
        $this->db->insert($this->table, $data);
    }

    public function delete($webinar_id, $id)
    {
        $this->switch_db($webinar_id);
        $this->db->delete($this->table, ['id' => $id]);
    }

    public function edit($webinar_id, $id, $data)
    {
        $this->switch_db($webinar_id);
        $data['date'] = date('Y-m-d H:i:s');
        $this->db->update($this->table, $data, ['id' => $id]);
    }

    public function update_status($webinar_id, $data)
    {
        $this->switch_db($webinar_id);
        if ($data['status'] == 1)
        {
            $tool = $this->get($webinar_id, $data['tool_id']);
            if (! empty($tool)) foreach (['banners','timers','polls','randoms','subscriptions'] as $table)
                $this->db->update($table, ['status' => 0], ['block_id' => $tool['block_id'],'status' => 1]);
        }
        $this->db->update($this->table, ['status' => $data['status']], ['id' => $data['tool_id']]);
    }
} 


You see this $this->switch_db($webinar_id); at the beginning of each method? It is used because of a separate db for each webinar (conference) instance. The problem that i need to pass $webinar_id param every time, which i get in controller as part of a URL. Duplicating code looks a bit awkward. I thought about constructor - BUT i can't pass an argument to it. Any suggestions?

Please, don't argue against "multi-db" decision - it can't be undone and has several advantages (smaller databases, better backup strategy).