Why does Django get_or_create cause this IntegrityError?

Question

I have a model that has this field on it:
token = models.CharField(max_length=32, default="", unique=True, null=False, db_index=True)

In the save() method I set this field to a 32 character random string using this method:

def save(self, *args, **kwargs):
    if (self.token is None or len(self.token) == 0):
        self.token = random_identifier()
    super(SessionPassthrough, self).save(*args, **kwargs)

def random_identifier(n=32):
    """ Generate a random identifier of length n. 

    From http://stackoverflow.com/questions/2257441/python-random-string-generation-with-upper-case-letters-and-digits"""
    return ''.join(random.choice(string.ascii_lowercase + string.digits) for x in range(n))

However I am getting this error whenever I try to create a new instance of the model:
IntegrityError: duplicate key value violates unique constraint "wakelytics_sessionpassthrough_token_key"

To create the instance I call this method:

@staticmethod
def for_session(session):
    sp, c = SessionPassthrough.objects.get_or_create(session=session)
    return sp

Does get_or_create() call the method's save() function before writing to the database? Answer: Yes

I get an IntegrityError whenever I call the method for the first time with a session, and continue getting the error for a few minutes. It then returns correctly. What is causing this?

You *do* know that even a random routine can pick a value that already exists... right? — Ignacio Vazquez-Abrams, Jun 16 '11 at 02:28
I am aware. But every time? and then it stops throwing an error? — Mantas Vidutis, Jun 16 '11 at 03:09
It's random. You don't know *when* it will pick a value not already in your database. — Ignacio Vazquez-Abrams, Jun 16 '11 at 03:12
But in that case it will return without the error _sometimes_, whereas now it reliably fails for the first few times/minutes and then succeeds. — Mantas Vidutis, Jun 16 '11 at 03:49

score 4 · Accepted Answer · answered Jun 16 '11 at 02:02

Yes.

https://docs.djangoproject.com/en/dev/ref/models/querysets/#get-or-create

From the docs:

The new object will be created roughly according to this algorithm:

defaults = kwargs.pop('defaults', {})
params = dict([(k, v) for k, v in kwargs.items() if '__' not in k])
params.update(defaults)
obj = self.model(**params)
obj.save()

Ignacio Vazquez-Abrams · Answer 2 · 2011-06-16T02:17:16.180

1

Of course it does. Part of its contract is that it will return a model that does exist, and in order to do that it has to save.

EDIT:

You're getting that error because the value already exists in the database.

edited Jun 16 '11 at 02:17

answered Jun 16 '11 at 02:03

Ignacio Vazquez-Abrams

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If it already exists, wouldnt the get_or_create simply return it? Why raise the error? – Mantas Vidutis Jun 16 '11 at 05:53
1

Because `session` is different, hence it's a separate row. – Ignacio Vazquez-Abrams Jun 16 '11 at 05:55

Why does Django get_or_create cause this IntegrityError?

2 Answers2