django admin many-to-many intermediary models using through= and filter_horizontal

Question

This is how my models look:

class QuestionTagM2M(models.Model):
    tag = models.ForeignKey('Tag')
    question = models.ForeignKey('Question')
    date_added = models.DateTimeField(auto_now_add=True)

class Tag(models.Model):
    description = models.CharField(max_length=100, unique=True)

class Question(models.Model):
    tags = models.ManyToManyField(Tag, through=QuestionTagM2M, related_name='questions')

All I really wanted to do was add a timestamp when a given manytomany relationship was created. It makes sense, but it also adds a bit of complexity. Apart from removing the .add() functionality [despite the fact that the only field I'm really adding is auto-created so it technically shouldn't interfere with this anymore]. But I can live with that, as I don't mind doing the extra QuestionTagM2M.objects.create(question=,tag=) instead if it means gaining the additional timestamp functionality.

My issue is I really would love to be able to preserve my filter_horizontal javascript widget in the admin. I know the docs say I can use an inline instead, but this is just too unwieldy because there are no additional fields that would actually be in the inline apart from the foreign key to the Tag anyway.

Also, in the larger scheme of my database schema, my Question objects are already displayed as an inline on my admin page, and since Django doesn't support nested inlines in the admin [yet], I have no way of selecting tags for a given question.

Is there any way to override formfield_for_manytomany(self, db_field, request=None, **kwargs) or something similar to allow for my usage of the nifty filter_horizontal widget and the auto creation of the date_added column to the database?

This seems like something that django should be able to do natively as long as you specify that all columns in the intermediate are automatically created (other than the foreign keys) perhaps with auto_created=True? or something of the like

Answer 1

There are ways to do this

• As provided by @obsoleter in the comment below : set QuestionTagM2M._meta.auto_created = True and deal w/ syncdb matters.

• Dynamically add date_added field to the M2M model of Question model in models.py

  class Question(models.Model):
      # use auto-created M2M model
      tags = models.ManyToMany(Tag, related_name='questions')
  
  
  # add date_added field to the M2M model
  models.DateTimeField(auto_now_add=True).contribute_to_class(
           Question.tags.through, 'date_added')
  

  Then you could use it in admin as normal ManyToManyField.
  In Python shell, use Question.tags.through to refer the M2M model.

  Note, If you don't use South, then syncdb is enough; If you do, South does not like this way and will not freeze date_added field, you need to manually write migration to add/remove the corresponding column.

• Customize ModelAdmin:

  1. Don't define fields inside customized ModelAdmin, only define filter_horizontal. This will bypass the field validation mentioned in Irfan's answer.
  2. Customize formfield_for_dbfield() or formfield_for_manytomany() to make Django admin to use widgets.FilteredSelectMultiple for the tags field.
  3. Customize save_related() method inside your ModelAdmin class, like

def save_related(self, request, form, *args, **kwargs):
    tags = form.cleaned_data.pop('tags', ())
    question = form.instance
    for tag in tags:
        QuestionTagM2M.objects.create(tag=tag, question=question)
    super(QuestionAdmin, self).save_related(request, form, *args, **kwargs)

• Also, you could patch __set__() of the ReverseManyRelatedObjectsDescriptor field descriptor of ManyToManyField for date_added to save M2M instance w/o raise exception.

Answer 2

From https://docs.djangoproject.com/en/dev/ref/contrib/admin/#working-with-many-to-many-intermediary-models

When you specify an intermediary model using the through argument to a ManyToManyField, the admin will not display a widget by default. This is because each instance of that intermediary model requires more information than could be displayed in a single widget, and the layout required for multiple widgets will vary depending on the intermediate model.

However, you can try including the tags field explicitly by using fields = ('tags',) in admin. This will cause this validation exception

'QuestionAdmin.fields' can't include the ManyToManyField field 'tags' because 'tags' manually specifies a 'through' model.

This validation is implemented in https://github.com/django/django/blob/master/django/contrib/admin/validation.py#L256

        if isinstance(f, models.ManyToManyField) and not f.rel.through._meta.auto_created:
            raise ImproperlyConfigured("'%s.%s' "
                "can't include the ManyToManyField field '%s' because "
                "'%s' manually specifies a 'through' model." % (
                    cls.__name__, label, field, field))

I don't think that you can bypass this validation unless you implement your own custom field to be used as ManyToManyField.