RelatedManager.get_or_create() does not work

[Antti Kaihola]

Given the following models:

class Poll(models.Model):
    question = models.CharField(maxlength=200)
    pub_date = models.DateTimeField('date published')

class Choice(models.Model):
    poll = models.ForeignKey(Poll)
    choice = models.CharField(maxlength=200)
    votes = models.IntegerField()

you can do:

>>> what, created = Poll.objects.get_or_create(question='What?', pub_date='2006-12-09')
>>> django = what.choice_set.create(choice='Django', votes=100)
>>> python, created = Choice.objects.get_or_create(poll=what, choice='Python', votes=200)

But for some reason this doesn't work:

>>> php, created = what.choice_set.get_or_create(choice='PHP', votes=1)

but gives the following error and traceback:

<class 'sqlite3.IntegrityError'> Traceback (most recent call last)

django/db/models/manager.py in get_or_create(self, **kwargs)
     68 
     69     def get_or_create(self, **kwargs):
---> 70         return self.get_query_set().get_or_create(**kwargs)
     71         
     72     def create(self, **kwargs):

django/db/models/query.py in get_or_create(self, **kwargs)
    238             params.update(defaults)
    239             obj = self.model(**params)
--> 240             obj.save()
    241             return obj, True
    242 

django/db/models/base.py in save(self)
    202                 cursor.execute("INSERT INTO %s (%s) VALUES (%s)" % \
    203                     (backend.quote_name(self._meta.db_table), ','.join(field_names),
--> 204                     ','.join(placeholders)), db_values)
    205             else:
    206                 # Create a new record with defaults for everything.

django/db/backends/util.py in execute(self, sql, params)
     10         start = time()
     11         try:
---> 12             return self.cursor.execute(sql, params)
     13         finally:
     14             stop = time()

django/db/backends/sqlite3/base.py in execute(self, query, params)
     90     def execute(self, query, params=()):
     91         query = self.convert_query(query, len(params))
---> 92         return Database.Cursor.execute(self, query, params)
     93 
     94     def executemany(self, query, param_list):

<class 'sqlite3.IntegrityError'>: testing_choice.poll_id may not be NULL

[Antti Kaihola]

The work-around is:

php, created = what.choice_set.get_or_create(poll=what, choice='PHP', votes=1)

But it obviously isn't DRY :)

[Brian Beck]

This bug also affects generic relation managers (see #2316}).

I tried to debug this, but it's pretty elusive. I think the solution might be to define get_or_create in the subclass anywhere create is also defined in the subclass. For instance, GenericRelatedObjectManager and ManyRelatedManager (among others, perhaps) both define a create method, but no get_or_create method. get_or_create is not defined to actually call its own class's create method (in QuerySet for instance, it duplicates create's code).

[Antti Kaihola]

Even worse, it seems that for many-to-many relations, the get_or_create method does create the related object but doesn't add a relation nor gives any error message.

For instance if you have model "Tag" with a field "name" and a model "BlogPost" with "tags=ManyToManyField(Tag)", then

myblogpost.tags.get_or_create(name='language_wars', defaults={'name':'language_wars'})

will create a Tag object with the correct name, but myblogpost.tags.all() is left unchanged.

In my opinion, the get_or_create method for both ForeignKeys and ManyToManyFields should be removed or throw a NotImplemented exception as long as it doesn't work correctly. The current functionality will confuse people.

[Jan Rademaker <j.rademaker@…>]

Patch + tests

[Jan Rademaker <j.rademaker@…>]

Basically, I just copied the get_or_create method from query.py (QuerySet.get_or_create).

[Chris Beaven]

I've been using this patch for a while now. I can't see a reason why it's not ready for checkin.

[Malcolm Tredinnick]

This doesn't have the same error checking as normal get_or_create() (if the save() fails). The patch needs more work and isn't ready for checkin yet. Also needs savepoint() additions to handle transactional support properly on PostgreSQL (again, as in the standard method).

The behaviour's inconsistent, so this is 1.0 if a proper patch can be produced.

[Gary Wilson]

(In [8415]) Fixed #3121 -- Made get_or_create() work for RelatedManager and ManyRelatedManager.

[Jacob]

Milestone 1.0 deleted