AttributeError when ParentalManyToManyField is specified with a through model

[m1kola]

Django throws AttributeError when ParentalManyToManyField is specified with a through model.

Demo with Wagtail:

from django.db import models
from wagtail.wagtailcore.models import Page
from modelcluster.fields import ParentalManyToManyField


class FooModel(models.Model):
    name = models.CharField(max_length=256)

    def __str__(self):
        return self.name


class BarToFoo(models.Model):
    foo = models.ForeignKey('appname.FooModel', blank=True, null=True)
    page = models.ForeignKey('BarPage', related_name='+')


class BarPage(Page):
    asset_programs = ParentalManyToManyField(
        to='appname.FooModel', through='appname.BarToFoo'
    )

Traceback:

File "/Users/mikalai/.virtualenvs/venv_name/lib/python3.4/site-packages/django/core/handlers/exception.py" in inner
  42.             response = get_response(request)

File "/Users/mikalai/.virtualenvs/venv_name/lib/python3.4/site-packages/django/core/handlers/base.py" in _get_response
  187.                 response = self.process_exception_by_middleware(e, request)

File "/Users/mikalai/.virtualenvs/venv_name/lib/python3.4/site-packages/django/core/handlers/base.py" in _get_response
  185.                 response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "/Users/mikalai/.virtualenvs/venv_name/lib/python3.4/site-packages/django/views/decorators/cache.py" in _cache_controlled
  43.             response = viewfunc(request, *args, **kw)

File "/Users/mikalai/.virtualenvs/venv_name/lib/python3.4/site-packages/wagtail/wagtailadmin/decorators.py" in decorated_view
  31.             return view_func(request, *args, **kwargs)

File "/Users/mikalai/.virtualenvs/venv_name/lib/python3.4/site-packages/wagtail/wagtailadmin/views/pages.py" in edit
  341.                 revision.publish()

File "/Users/mikalai/.virtualenvs/venv_name/lib/python3.4/site-packages/wagtail/wagtailcore/models.py" in publish
  1539.         page.save()

File "/Users/mikalai/.pyenv/versions/3.4.6/lib/python3.4/contextlib.py" in inner
  30.                 return func(*args, **kwds)

File "/Users/mikalai/.virtualenvs/venv_name/lib/python3.4/site-packages/wagtail/wagtailcore/models.py" in save
  477.         result = super(Page, self).save(*args, **kwargs)

File "/Users/mikalai/.virtualenvs/venv_name/lib/python3.4/site-packages/modelcluster/models.py" in save
  209.             getattr(self, field).commit()

File "/Users/mikalai/.virtualenvs/venv_name/lib/python3.4/site-packages/modelcluster/fields.py" in commit
  442.                 original_manager.add(*items_to_add)

File "/Users/mikalai/.virtualenvs/venv_name/lib/python3.4/site-packages/django/db/models/fields/related_descriptors.py" in add
  876.                     (opts.app_label, opts.object_name)

Exception Type: AttributeError at /admin/pages/133/edit/
Exception Value: Cannot use add() on a ManyToManyField which specifies an intermediary model. Use appname.BarToFoo's Manager instead.

Django==1.10.7
wagtail==1.11.1

[gasman]

Is this feature really needed, or can the same thing be done with a ParentalKey on the through model (i.e. the way we 'faked' M2M relationships before ParentalManyToManyField was implemented)...?

[m1kola]

This feature is really useful for projects with models that were implemented before modelcluster/Wagtail introduced M2M support. So models like

class BarToFoo(models.Model):
    foo = models.ForeignKey('appname.FooModel', blank=True, null=True)
    page = models.ParentalKey('BarPage', related_name='something')

can be modified to become:

class BarToFoo(models.Model):
    foo = models.ForeignKey('appname.FooModel', blank=True, null=True)
    page = models.ForeignKey('BarPage', related_name='+')

Then you just need to add something = ParentalManyToManyField(..., through='appname.BarToFoo') and replace InlinePanel in the "target" model with FieldPanel. No need to write data migrations.

It also allows developers to add additional data into through models.

[mjpieters]

A through model is a requirement when you want to record additional information about the relationship.

I need to record a rich text description per many-to-many relationship, the (source, target) tuples must be unique, and I need to put minimum and maximum limits on the relationship (each source needs to have at least 1, at most 8 such relationships recorded).

I guess I can achieve this with an inline panel and a full-blown model with a unique_together constraint, and some way to limit the available choices based on what's already present when editing? The latter would become rather hackish, I fear.

[mihow]

The syntax for ParentalManyToManyField(to=FooModel, through=BarToFoo) seems more Django-like rather than only defining the intermediary model and setting a ParentalKey as seen in the Wagtail examples.

Also it looks like Django just merged better support for updating m2m relationships that have an intermediary model as well: django/django#8981. So for the example in the ticket description you will be able to use bar_page.asset_programs.add(foo) even with a through model!