Begin Transaction Not Work

Sorry everyone i need help about using trans_begin() to handle mutiple insert with multi table, im try  to insert data "Tagihan" after insert data 'Santri' but ihave a problem. in query insert data "Santri" is success but not in "Tagihan" because in "Tagihan" field "biaya_id" is unknown..

i mean with trans_begin the Data will be rollback or not insert when getting error in query...

PHP Code:

$this->db->trans_begin();

        $this->db->set($data)->insert('santri');

        $santri_id = $this->db->insert_id();
        $kelas_id  = $data['kelas_id'];

        $this->db->select('biaya_id, nominal');
        $this->db->from('biaya_per_kelas');
        $this->db->where('kelas_id', $kelas_id);

        $query  = $this->db->get();

        if ($query->num_rows() > 0){
            foreach ($query->result_array() as $biaya) {
                $this->db->set('santri_id', $santri_id);
                $this->db->set('biaya_id', $biaya['biaya_id']);
                $this->db->set('nominal', $biaya['nominal']);
                $this->db->insert('tagihan');
            }
        }

        if ($this->db->trans_status() === FALSE) {
            $this->db->trans_rollback();

            return true;
        } else {
            $this->db->trans_commit();

            return false;
        } 


Does this query in your script return expected values?

PHP Code:

$this->db->select('biaya_id, nominal');
$this->db->from('biaya_per_kelas');
$this->db->where('kelas_id', $kelas_id);
$query  = $this->db->get();
print_r($query->result());