#18896: get_or_create breaks for ManyToMany
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Reporter: mattlong | Owner: nobody
Type: Bug | Status: new
Component: Database layer (models, ORM) | Version: 1.4
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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Using get_or_create through a ManyToMany field results in an integrity
error if the object being queried for already exists but is not yet
associated with the parent object:
{{{
class Tag(models.Model):
text = models.CharField(max_length=256, unique=True)
class Thing(models.Model):
name = models.CharField(max_length=256)
tags = models.ManyToManyField(Tag)
#create and save a Tag
Tag.objects.create(text='foo')
#create and save a Thing
a_thing = Thing.objects.create(name='a')
#get the previously created Tag and have it associated with a_thing
a_thing.tags.get_or_create(text='foo') #should get
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 text is not unique
}}}
To summarize, if a Tag with text 'foo' exists but is not yet associated
with a given Thing instance, using .tags.get_or_create(text='foo') raises
an IntegrityError since it tries to re-create the same Tag.
I'm not familiar with the Django ORM source code, but I've traced the
issue to ManyRelatedManager's get_query_set method always including its
core_filters. 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.
In this case, I would expect .tags.get_or_create(...) to just add/save the
association between Thing and Tag and return the existing Tag.
--
Ticket URL: <https://code.djangoproject.com/ticket/18896>
Django <https://code.djangoproject.com/>
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