php - CodeIgniter Transaction not working properly -

i have function in model supposed run 2 queries in transaction, update query not working.

public function delete($id = null)  {     if($id) {         $delete = "delete borrowed_books          id = '$id'; ";          $mod="update `books` b               inner join `borrowed_books`               set b.nr_copies=b.nr_copies+1               b.id_book=a.id_book , a.id = '$id'; ";          $this->db->trans_start();         $this->db->query($delete);         $this->db->query($mod);         $this->db->trans_complete();         if ($this->db->trans_status() === false) {             return false;         } else {             return true;         }     }     } 

a quickfix might executing update before deleting;

public function delete($id = null) {     if ($id) {         $delete = "delete borrowed_books          id = '$id'; ";          $mod="update `books` b               inner join `borrowed_books`               set b.nr_copies=b.nr_copies+1               b.id_book=a.id_book , a.id = '$id'; ";          $this->db->trans_start();         // execute update         $this->db->query($mod);         // delete         $this->db->query($delete);         $this->db->trans_complete();         if ($this->db->trans_status() === false) {             return false;         } else {             return true;         }     }    } 

the update query condition; looking data not existent because deleted it. if update (then query finds result assuming condition correct), update..