cara casting tipe data di codeigniter

gan mau tanya gimana ya cara inner join casting/convert di codeigniter.

$this->db->select('t_trx_activity.activity_id,
		t_trx_activity.nik,t_trx_activity.start_date,
		t_trx_activity.end_date,t_trx_activity.status,t_mtr_employee.full_name');
		$this->db->from('t_trx_activity');
		$this->db->where_in('status',$status);
		//casting join table
		$this->db->join('t_mtr_activity_employee','t_trx_activity.nik = CAST('t_mtr_employee.nik'),'inner');