linq更新操作数据库值没变化

var supplie = db.tb_supplies.Where(s => s.oid == id).SingleOrDefault();
        if (supplie!=null)
        {
            supplie.otop = top;
            supplie.oleft = left;
            db.SubmitChanges();
            return "success!";
        }
为什么执行之后数据库的值没有改变。

var supplie = db.tb_supplies.Where(s => s.oid == id).SingleOrDefault();
        if (supplie!=null)
        {
            supplie.otop = top;
            supplie.oleft = left;
            [color=#FF0000]db.tb_supplies.InsertOnSubmit(supplie);[/color]
            db.SubmitChanges();
            return "success!";
        }

------解决方案--------------------

解决就好
代码还有简化的空间：
var supplie = db.tb_supplies.SingleOrDefault(s => s.oid == id);