$MODEL = getcache('model','commons');//获取表名
$tablename = $this->db->table_name = $this->db->db_tablepre.$MODEL[$modelid]['tablename'];//v9_download
$id=8;
$rs=$this->db->get_one(array('id'=>$id));
$amount=$rs['amount'];
$sql = array('down_amount'=>$amount+1,);//把sql定义成一个数组。填写修改的字段,后面是值
$this->db->update($sql, array('id'=>$id));//修改的约束条件