什么会导致Linq-to-SQL对象忽略新值

也许我遗漏了一些明显的东西，或者可能发生了一些险恶的事情，但这对我来说似乎很奇怪。我有一个Linq-to-SQL数据上下文，我正在尝试更新记录上的字段。更新不起作用，SQL事件探查器跟踪显示没有向数据库发出更新命令。因此，我缩小了代码的确切行数，并删除了各种变量，试图简化。

在where子句不变的情况下，调试器中现在的情况如下：

var foo = db.f_pp_Budgets
            .Where(b => b.SeasonKey == seasonID)
            .Where(b => b.Business == businessUnit)
            .Where(b => b.Level == "R")
            .Where(b => b.CodeKey == region.Key)
            .Single().Trade;
// foo is now set to 1
db.f_pp_Budgets
            .Where(b => b.SeasonKey == seasonID)
            .Where(b => b.Business == businessUnit)
            .Where(b => b.Level == "R")
            .Where(b => b.CodeKey == region.Key)
            .Single().Trade = region.Budget.Trade;
// region.Budget.Trade was (and still is) 2
// so the underlying Linq to SQL object should have a 2 in it, right?
var baz = db.f_pp_Budgets
            .Where(b => b.SeasonKey == seasonID)
            .Where(b => b.Business == businessUnit)
            .Where(b => b.Level == "R")
            .Where(b => b.CodeKey == region.Key)
            .Single().Trade;
// baz is set to 1.  Shouldn't it be 2?

由于该值不变，因此调用db.SubmitChanges()自然不会向数据库发送任何内容。

有人能想到价值不会改变的原因吗？

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更新代码：（回应@Hogan的回答）

var myObj = db.f_pp_Budgets
              .Where(b => b.SeasonKey == seasonID)
              .Where(b => b.Business == businessUnit)
              .Where(b => b.Level == "R")
              .Where(b => b.CodeKey == region.Key)
              .Single();
var foo = myObj.Trade;
// foo is now set to 1
myObj.Trade = region.Budget.Trade;
// region.Budget.Trade was (and still is) 2
var baz = myObj.Trade;
// baz is now set to 2

正在更新对象的值。但是，db.SubmitChanges()仍然没有向数据库发送任何update语句。

 var myobj = db.f_pp_Budgets
        .Where(b => b.SeasonKey == seasonID)
        .Where(b => b.Business == businessUnit)
        .Where(b => b.Level == "R")
        .Where(b => b.CodeKey == region.Key)
        .Single();
myobj.Trade = region.Budget.Trade;

 5+7 = 4
 obj.val + obj2.val = 4