在实体框架中以事务方式获取或创建数据库对象
我正在使用 Entity Framework 4.1，并且有一个看似简单的要求：我想通过唯一键获取一个实体，或者如果它尚不存在，则创建它：
var user = db.Users.SingleOrDefault(u => u.Sid == sid);
if (user != null)
return user;
user = new User(sid);
db.Users.Add(user);
通常这工作正常，但是当我运行一堆时一起测试（使用 MSTest），其中之一始终失败，并显示“序列包含多个元素”。当我单独运行该测试时，它工作得很好。
问题似乎很明显：多个线程同时调用上述代码，每个线程创建一个新的用户行。但解决办法是什么？
当然，正确的解决方案是事务，但我就是无法让它发挥作用。如果我启动一个普通的 DbTransaction，EF 将不会使用它。如果我使用 TransactionScope，它要么没有效果（发生相同的错误），要么 EF 尝试启动分布式事务但失败，即使我遵循 有关首先打开连接的建议。
这确实令人沮丧，因为对于普通的旧 SQL 来说，这是一件非常琐碎的事情：开始事务、SELECT、INSERT、提交事务。我怎样才能让它在 EF 中工作？它不必使用事务——无论它如何工作。
I'm using Entity Framework 4.1 and have a seemingly simple requirement: I want to either get an entity by a unique key, or if it doesn't already exist, create it:
var user = db.Users.SingleOrDefault(u => u.Sid == sid);
if (user != null)
return user;
user = new User(sid);
db.Users.Add(user);
Normally this works fine, but when I run a bunch of tests together (using MSTest) one of them consistently fails with "Sequence contains more than one element". When I run that test by itself it works fine.
The problem seems obvious: multiple threads are calling the above code concurrently and each create a new User row. But what is the solution?
The proper solution is a transaction, of course, but I just cannot get it to work. EF won't use a normal DbTransaction if I start one. If I use a TransactionScope it either has no effect (the same error occurs) or EF tries and fails to start a distributed transaction, even if I follow the advice about opening a connection first.
This is really frustrating, because it is such a trivial thing to do with plain old SQL: begin transaction, SELECT, INSERT, commit transaction. How can I get this to work in EF? It doesn't have to be using transactions - whatever makes it work.
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如果您的数据库对
Sid有UNIQUE约束，第一个语句（唯一可能导致您描述的错误的语句）将永远不会失败。是吗？它应该。这是确保sid真正全局唯一的唯一方法。The first statement (the only one which could cause the error you describe) will never fail if your DB has a
UNIQUEconstraint onSid. Does it? It should. That's the only way to make sure that thesidis truly, globally unique.