get_or_create（）线程是否安全 [英] Is get_or_create() thread safe

问题描述

我有一个只能使用 get_or_create（session = session）访问的Django模型，其中session是另一个Django模型的外键。

I have a Django model that can only be accessed using get_or_create(session=session), where session is a foreign key to another Django model.

由于我只能通过 get_or_create（）进行访问，我可以想像，我只会有一个实例与会话的关键。但是，我已经发现了多个具有密钥的实例到同一个会话。发生什么事？这是一个竞争条件，还是 get_or_create（）原子运算？

Since I am only accessing through get_or_create(), I would imagine that I would only ever have one instance with a key to the session. However, I have found multiple instances with keys to the same session. What is happening? Is this a race condition, or does get_or_create() operate atomically?

推荐答案

p> Actualy它不是线程安全的，您可以查看QuerySet对象的get_or_create方法的代码，其基本原理如下：

Actualy it's not thread-safe, you can look at the code of the get_or_create method of the QuerySet object, basicaly what it does is the following :

try:
    return self.get(**lookup), False
except self.model.DoesNotExist:
    params = dict([(k, v) for k, v in kwargs.items() if '__' not in k])
    params.update(defaults)
    obj = self.model(**params)
    sid = transaction.savepoint(using=self.db)
    obj.save(force_insert=True, using=self.db)
    transaction.savepoint_commit(sid, using=self.db)
    return obj, True

所以两个线程可能会弄清楚实例在数据库中不存在，并开始创建一个新的，然后连续保存。

So two threads might figure-out that the instance does not exists in the DB and start creating a new one, before saving them consecutively.

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