On Thu, Mar 25, 2010 at 3:47 PM, Malcolm Tredinnick
<malc...@pointy-stick.com> wrote:
> On Thu, 2010-03-25 at 11:22 +0530, Kenneth Gonsalves wrote:
>> hi,
>>
>> I have a model like this:
>>
>> name - unique
>> slno - not null
>> mode - not null
>>
>> If the instance does not exist and I give all three values to get_or_create,
>> it works
>>
>> if the instance exists, and I give values for slno and mode which are
>> different
>> from the existing ones - I get a programming error as get_or_create tries to
>> create a new instance and falls foul of the unique constraint on name. It
>> works if I only give the value for name - it gets the instance and then I can
>> add the new values and save
>
> I don't understand what you are saying here. If the instance exists (as
> in, the filter parameters passed to get would return a single item),
> that instance is returned and the default parameters are never
> considered. So the values you pass for slno and mode are irrelevant. If
> the equivalent get() call would not return anything, then the instance
> does not exist and it's clear that the default parameter must contain
> correct data that can be used to create a new object -- including not
> violating any unique constraints. Perhaps you could show the code call
> you are making, rather than writing it out as a sentence.
>
> In short: get_or_create() first runs the get(). If that returns
> something, it is what is returned. If the get() fails with no object
> being present, then the instance DOES NOT exist and a new one is
> created, with all the same requirements as if you'd created a new object
> from scratch (e.g. no unique constraint violations).
>
> Malcolm
>
>
He is describing this situation:
>>> User.objects.get(username='tevans...@googlemail.com')
<User: tevans...@googlemail.com>
>>> User.objects.get_or_create(username='tevans...@googlemail.com')
(<User: tevans...@googlemail.com>, False)
>>> User.objects.get(username='tevans...@googlemail.com').first_name
u'Tom'
>>> User.objects.get(username='tevans...@googlemail.com', first_name='Thomas')
Traceback (most recent call last):
File "<console>", line 1, in <module>
File "....site-packages/django/db/models/manager.py", line 119, in get
return self.get_query_set().get(*args, **kwargs)
File "....django/db/models/query.py", line 305, in get
% self.model._meta.object_name)
DoesNotExist: User matching query does not exist.
>>> User.objects.get_or_create(username='tevans...@googlemail.com',
>>> first_name='Thomas')
Traceback (most recent call last):
File "<console>", line 1, in <module>
File "....site-packages/django/db/models/manager.py", line 122, in
get_or_create
return self.get_query_set().get_or_create(**kwargs)
File "....site-packages/django/db/models/query.py", line 343, in get_or_create
raise e
IntegrityError: (1062, "Duplicate entry '' for key 2")
IE, where Model.objects.get(**kwargs) does not return an object, but
Model.objects.create(**kwargs) would fail, which is fixed by knowing
about the 'defaults' kwarg to get_or_create:
>>> User.objects.get_or_create(username='tevans...@googlemail.com',
... defaults={'first_name': 'Thomas', })
(<User: tevans...@googlemail.com>, False)
Cheers
Tom
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