Many-to-many self-referential intermediates in admin: <Friendship> has more than 1 ForeignKey to <Person>

[Boo]

This URL ​http://127.0.0.1:8000/admin/foo/person/add/ returns traceback:

Environment:

Request Method: GET
Request URL: http://127.0.0.1:8000/admin/foo/person/add/
Django Version: 1.0-alpha-SVN-8199
Python Version: 2.5.2
Installed Applications:
['django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.sites',
 'django.contrib.admin',
 'testcase.foo']
Installed Middleware:
('django.middleware.common.CommonMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.middleware.doc.XViewMiddleware')


Traceback:
File "/opt/local/lib/python2.5/site-packages/django/core/handlers/base.py" in get_response
  87.                 response = callback(request, *callback_args, **callback_kwargs)
File "/opt/local/lib/python2.5/site-packages/django/contrib/admin/sites.py" in root
  156.                 return self.model_page(request, *url.split('/', 2))
File "/opt/local/lib/python2.5/site-packages/django/views/decorators/cache.py" in _wrapped_view_func
  44.         response = view_func(request, *args, **kwargs)
File "/opt/local/lib/python2.5/site-packages/django/contrib/admin/sites.py" in model_page
  173.         return admin_obj(request, rest_of_url)
File "/opt/local/lib/python2.5/site-packages/django/contrib/admin/options.py" in __call__
  259.             return self.add_view(request)
File "/opt/local/lib/python2.5/site-packages/django/contrib/admin/options.py" in add_view
  504.             for FormSet in self.get_formsets(request):
File "/opt/local/lib/python2.5/site-packages/django/contrib/admin/options.py" in get_formsets
  340.             yield inline.get_formset(request, obj)
File "/opt/local/lib/python2.5/site-packages/django/contrib/admin/options.py" in get_formset
  743.             extra=self.extra, max_num=self.max_num)
File "/opt/local/lib/python2.5/site-packages/django/forms/models.py" in inlineformset_factory
  479.     fk = _get_foreign_key(parent_model, model, fk_name=fk_name)
File "/opt/local/lib/python2.5/site-packages/django/forms/models.py" in _get_foreign_key
  464.             raise Exception("%s has more than 1 ForeignKey to %s" % (model, parent_model))

Exception Type: Exception at /admin/foo/person/add/
Exception Value: <class 'testcase.foo.models.Friendship'> has more than 1 ForeignKey to <class 'testcase.foo.models.Person'>

Please, see the testcase to reproduce this bug.

[Boo]

TestCase to repeat the problem

[MilosU]

There is no problem actually, you only need to define the fkey that admin app should use for the relation. If you have model like this (your testcase):

class Person(models.Model):

name = models.CharField(max_length=20)
friends = models.ManyToManyField('self', through='Friendship', related_name='friend_of', symmetrical=False)

def unicode(self):

return self.name

class Friendship(models.Model):

first = models.ForeignKey(Person, related_name='rel_from_set')
second = models.ForeignKey(Person, related_name='rel_to_set')
date_friended = models.DateTimeField(auto_now_add=True)

def unicode(self):

return u'%s

%s' % (self.first.name, self.second.name)

You need to specify fk_name attr on inline object:

class FriendshipInline(admin.TabularInline):

model = Friendship
fk_name = 'first'

Similarly could could edit the second side of the relation like this on a single page:
class SecondFriendshipInline(admin.TabularInline):

model = Friendship
fk_name = 'second'

Your PersonAdmin would be then like this:

class PersonAdmin(admin.ModelAdmin):

inlines = [FriendshipInline, SecondFriendshipInline]

admin.site.register(Person, PersonAdmin)

This subtle config addon is however not mentioned currently in the docs, you need to go to the source.

[MilosU]

Hmm.. Actually, this is already documented here:

​http://www.djangoproject.com/documentation/admin/#working-with-a-model-with-two-or-more-foreign-keys-to-the-same-parent-model

I guess this was not there 5 hours ago..

--
Milos

[anonymous]

The documentation seems to have been moved from underneath Milos' link. Here's an updated link:

​https://docs.djangoproject.com/en/dev/ref/contrib/admin/#working-with-a-model-with-two-or-more-foreign-keys-to-the-same-parent-model