Unexpected behavior with ManyRelatedManager get_or_create()

[Fred Bartle]

I've noticed 2 things happening when using a ManyRelatedManager get_or_create method:

1. The new related object is created but not added to the current object. Looking at django/db/models/fields/related.py, get_or_create is not overridden() in the ManyRelatedManager definition, but create() is to add the new object.

2. Using get_or_create() to add a new object that already exists regardless of whether or not it's linked to the current object results in a related object DoesNotExist exception.

from django.db import models

class Controller(models.Model):
    name = models.CharField(max_length=30, unique=True)

    def __unicode__(self):
        return self.name


class ControllerGroup(models.Model):
    name = models.CharField(max_length=30, unique=True)
    controllers = models.ManyToManyField(Controller)

    def __unicode__(self):
        return self.name


# create a controller
>>> c1 = Controller(name='controller1').save()

# create a controller group
>>> cg = ControllerGroup(name='my group').save()

# 'add' using get_or_create()
>>> cg.controllers.get_or_create(name='controller1')
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/local/django/src/trunk/django/db/models/manager.py", line 84, in get_or_create
    return self.get_query_set().get_or_create(**kwargs)
  File "/usr/local/django/src/trunk/django/db/models/query.py", line 335, in get_or_create
    return self.get(**kwargs), False
  File "/usr/local/django/src/trunk/django/db/models/query.py", line 300, in get
    % self.model._meta.object_name)
DoesNotExist: Controller matching query does not exist.

# trying 'adding' a controller that doesn't exist
>>> cg.controllers.get_or_create(name='controller2')
(<Controller: controller2>, True)

# looks like it's been added, let's check
>>> cg.controllers.all()
[]

[Fred Bartle]

override get_or_create() in ManyRelatedManager

[Fred Bartle]

Code should actually read:

# create a controller
>>> c1 = Controller(name='controller1')
>>> c1.save()

# create a controller group
>>> cg = ControllerGroup(name='my group')
>>> cg.save()

[Fred Bartle]

My patch is not correct, I will spend some time reviewing it.

[Fred Bartle]

fixed missing import, used correct get() method

[Alex Gaynor]

I believe this is a dupe of #3121(which was fixed in r8415), if this is incorrect, please reopen.

[Jacob]

Milestone 1.0 deleted