Django: IntegrityError during Many To Many add()

Question

We run into a known issue in django:

IntegrityError during Many To Many add()

There is a race condition if several processes/requests try to add the same row to a ManyToManyRelation.

How to work around this?

Envionment:

• Django 1.9
• Linux Server
• Postgres 9.3 (An update could be made, if necessary)

Details

How to reproduce it:

my_user.groups.add(foo_group)

Above fails if two requests try to execute this code at once. Here is the database table and the failing constraint:

myapp_egs_d=> \d auth_user_groups
  id       | integer | not null default ...
  user_id  | integer | not null
  group_id | integer | not null
Indexes:
           "auth_user_groups_pkey" PRIMARY KEY, btree (id)
fails ==>  "auth_user_groups_user_id_group_id_key" UNIQUE CONSTRAINT,
                                            btree (user_id, group_id)

Environment

Since this only happens on production machines, and all production machines in my context run postgres, a postgres only solution would work.

Answer 1

Can the error be reproduced?

Yes, let us use the famed Publication and Article models from Django docs. Then, let's create a few threads.

import threading
import random

def populate():

    for i in range(100):
        Article.objects.create(headline = 'headline{0}'.format(i))
        Publication.objects.create(title = 'title{0}'.format(i))

    print 'created objects'


class MyThread(threading.Thread):

    def run(self):
        for q in range(1,100):
            for i in range(1,5):
                pub = Publication.objects.all()[random.randint(1,2)]
                for j in range(1,5):
                    article = Article.objects.all()[random.randint(1,15)]
                    pub.article_set.add(article)

            print self.name


Article.objects.all().delete()
Publication.objects.all().delete()
populate()
thrd1 = MyThread()
thrd2 = MyThread()
thrd3 = MyThread()

thrd1.start()
thrd2.start()
thrd3.start()

You are sure to see unique key constraint violations of the type reported in the bug report. If you don't see them, try increasing the number of threads or iterations.

Is there a work around?

Yes. Use through models and get_or_create. Here is the models.py adapted from the example in the django docs.

class Publication(models.Model):
    title = models.CharField(max_length=30)

    def __str__(self):              # __unicode__ on Python 2
        return self.title

    class Meta:
        ordering = ('title',)

class Article(models.Model):
    headline = models.CharField(max_length=100)
    publications = models.ManyToManyField(Publication, through='ArticlePublication')

    def __str__(self):              # __unicode__ on Python 2
        return self.headline

    class Meta:
        ordering = ('headline',)

class ArticlePublication(models.Model):
    article = models.ForeignKey('Article', on_delete=models.CASCADE)
    publication = models.ForeignKey('Publication', on_delete=models.CASCADE)
    class Meta:
        unique_together = ('article','publication')

Here is the new threading class which is a modification of the one above.

class MyThread2(threading.Thread):

    def run(self):
        for q in range(1,100):
            for i in range(1,5):
                pub = Publication.objects.all()[random.randint(1,2)]
                for j in range(1,5):
                    article = Article.objects.all()[random.randint(1,15)]
                    ap , c = ArticlePublication.objects.get_or_create(article=article, publication=pub)
            print 'Get  or create', self.name

You will find that the exception no longer shows up. Feel free to increase the number of iterations. I only went up to a 1000 with get_or_create it didn't throw the exception. However add() usually threw an exception with in 20 iterations.

Why does this work?

Because get_or_create is atomic.

This method is atomic assuming correct usage, correct database configuration, and correct behavior of the underlying database. However, if uniqueness is not enforced at the database level for the kwargs used in a get_or_create call (see unique or unique_together), this method is prone to a race-condition which can result in multiple rows with the same parameters being inserted simultaneously.

Update: Thanks @louis for pointing out that the through model can in fact be eliminated. Thuse the get_or_create in MyThread2 can be changed as.

ap , c = article.publications.through.objects.get_or_create(
            article=article, publication=pub)