class user
public function setGroup($id)
    {   $this->_db->get('groups',array('id','>','0'));
        $results = $this->_db->results();
        $exist=false;
        foreach ($results as $result) {
            echo $result->id,'<br>';
            echo $id,'<br>';
            if ($result->id===$id){
                $exist=true;}
        }
        if (!$exist){
            echo "not done ";
            return false ;
        }else{
            echo 'done';
            try {
                $this->update(array(
                    'group'=>$id));
            }catch (Exception $e ){
                die($e->getMessage());
            }

        }
    }
************************
class user
 public function update($fields = array(),$id=null){
        if (!$id && $this->isLoggedIn){
            $id=$this->data()->id;
        }
        if (!$this->_db->update('users',$id,$fields)){
            throw new Exception('There was a problem updating');
        }
    }
*******************
class DB
public function update($table,$id,$fields){
        $set='';
        $x=1;
        foreach ($fields as $name=>$value ){
            $set .= "{$name}=?";
                if ($x<count($fields)){
                    $set .=',';
                }
                $x++;
        }
        $sql  = "UPDATE {$table} SET {$set} WHERE id = {$id}";
        echo $sql ;
        var_dump($fields);
        if(!$this->query($sql,$fields)->error()){
            return true ;
        }
        return false ;
    }