Re: ManyToManyField and get_or_create

Bump?

Almost exactly 4 years later, I've ran into this exact same issue in Django 1.4. Attempting to use get_or_create through a ManyToMany field results in an integrity error if the object already exists but is not yet associated with the parent object. To use the OP's example, if a Tag with name "foo" exists but is not yet associated with a given Thing instance, using .tags.get_or_create(name='foo') will indeed raise an IntegrityError:

foo_tag = Tag(name='foo')

foo_tag.save()

a_thing = Thing(name='a')

a_thing.save() 

a_thing.tags.get_or_create(name='foo')

Traceback (most recent call last):

  File "<input>", line 1, in <module>

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/related.py", line 616, in get_or_create

    super(ManyRelatedManager, self.db_manager(db)).get_or_create(**kwargs)

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/manager.py", line 134, in get_or_create

    return self.get_query_set().get_or_create(**kwargs)

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 449, in get_or_create

    obj.save(force_insert=True, using=self.db)

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/base.py", line 463, in save

    self.save_base(using=using, force_insert=force_insert, force_update=force_update)

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/base.py", line 551, in save_base

    result = manager._insert([self], fields=fields, return_id=update_pk, using=using, raw=raw)

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/manager.py", line 203, in _insert

    return insert_query(self.model, objs, fields, **kwargs)

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 1576, in insert_query

    return query.get_compiler(using=using).execute_sql(return_id)

  File "/usr/local/lib/python2.7/dist-packages/django/db/models/sql/compiler.py", line 910, in execute_sql

    cursor.execute(sql, params)

  File "/usr/local/lib/python2.7/dist-packages/django/db/backends/util.py", line 40, in execute

    return self.cursor.execute(sql, params)

  File "/usr/local/lib/python2.7/dist-packages/django/db/backends/sqlite3/base.py", line 337, in execute

    return Database.Cursor.execute(self, query, params)

IntegrityError: column name is not unique

I've traced the problem to being that the ManyRelatedManager includes its core_filters in its get_query_set method. This results in the "get" portion of get_or_create to only return a hit if the Tag exists and is already associated to the calling Thing instance. Given the nature of a many-to-many relationship, it should not be a requirement that a Tag already be linked to the calling Thing for get_or_create to find it; it should be enough that the Tag simply exists. All that should happen in that scenario is that the (existing) Tag's relationship with the calling Thing be created/saved.

On Monday, September 1, 2008 8:04:00 PM UTC-7, Cap wrote:

I'm having problems using get_or_create with a ManyToManyField and I'm
not sure whether the problem is me or Django.

I have two models:

class Tag(models.Model):
    name = models.CharField(max_length=256, unique=True)

class Thing(models.Model):
    name = models.CharField(max_length=256)
    tags = models.ManyToManyField(Tag)

When I try to add a tag that already exists, as:

a = Thing(name='a')
a.save()
a.tags.get_or_create(name='foo')

I get sqlite3.IntegrityError: column name is not unique.

But when I do it like so:

a = Thing(name='a')
a.save()
foo, created = Tag.objects.get_or_create(name='foo')
a.tags.add(foo)

I have no problems.

I noticed that http://code.djangoproject.com/ticket/3121 seemed to
address something like this, so I pulled the latest Django (8834), but
the problems remains.

Am I doing it wrong?

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