weird bug on database model [ci 1.6.2]

[eluser]adwin[/eluser]
Hi .. I found a bug that not always happened in my model

i use ci 1.6.2

here is my code

Code:

function save()
        {
            $this->db->set('measureid',$this->measureid);
            $this->db->set('goodsid',$this->goodsid);
            $this->db->set('poid',$this->poid);
            $this->db->set('qty',$this->qty);
            $this->db->set('qty0',$this->qty0);
            
            $this->db->set('unitprice',$this->unitprice);
            $this->db->set('discount',$this->discount);
            $this->db->set('total', $this->calculatetotal());
            
            $this->db->trans_begin();
            $status = '';  
            if($this->id != 0)
            {
                $this->db->where('id',$this->id);
                $status = $this->db->update($this->_table);
            }else
            {
                $status = $this->db->insert($this->_table);
                $this->id = $this->db->insert_id() ;
            }
            
            if(!$this->checkstatus($status,$this->_table)){
                echo $this->goodsid.' <br/>';
                echo $this->db->last_query();
                return false;
            }

            if(!$this->updateHeader($this->poid))
                return false;      

            if ($this->db->trans_status()===FALSE){
              return $this->rolllback();
            } else {
              $this->db->trans_commit();
              return true;
            }
        }

as you can see, there is

Code:

$this->db->set('measureid',$this->measureid);
$this->db->set('goodsid',$this->goodsid);

on the top of the db->set();

measureid and goodsid is linked with other table (foreign keys)

when I did insert it produce the error because measureid or goodsid not included into the insert sql.

here is the error:

Code:

Cannot add or update a child row: a foreign key constraint fails (`ci/podt`, CONSTRAINT `podt_ibfk_2` FOREIGN KEY (`goodsid`) REFERENCES `mtgoods` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION)
SQL : INSERT INTO `podt` (`measureid`, `qty`, `unitprice`, `discount`, `total`) VALUES ('1', 20, '30000', '300', 599700)

as you can see on my code, i have set the goodsid and measureid right ?