Issue with tags in a nested snippet model

Issue Summary

I have a simple Quiz snippet whith questions and answers. The answers are nested and in relation to the question. The answers also have tags configured to them. On saving this causes an exception because of the nested tags and formsets. [‘ManagementForm data is missing or has been tampered with’]

Steps to Reproduce

@register_snippet
@python_2_unicode_compatible
class QuizQuestion(ClusterableModel):
    question = models.CharField(max_length=255)

    panels = [
        FieldPanel('question', classname="full"),
        InlinePanel('quiz_answer_list', label="Answers"),
    ]

    def __str__(self):
        return self.vraag


class QuizAnswerTag(TaggedItemBase):
    content_object = ParentalKey('tools.QuizAnswer', on_delete=models.CASCADE, related_name='quiz_tagged_items')


class QuizAnswer(ClusterableModel, Orderable):
    question = ParentalKey(QuizQuestion, related_name='quiz_answer_list')
    answer = models.CharField(max_length=255)
    tags = ClusterTaggableManager(through=QuizAnswerTag, blank=True)

    panels = [
        FieldPanel('answer'),
        FieldPanel('tags'),
    ]

Create a question with some answers. Saving will throw the exception.

• I have confirmed that this issue can be reproduced as described on a fresh Wagtail project: yes

Technical details

Python: 3.6.8 Django 2.1.9 Wagtail version: 2.5.x

[gasman]

PR created: https://github.com/wagtail/django-modelcluster/pull/119

I’ve made it so that the ‘ignore missing formsets’ behaviour only applies when formsets are not specified explicitly - this way, when we implement the proper fix of specifying the formsets on the Wagtail side, the ‘hackiness’ (i.e. ignoring a potentially legitimate error case) is confined to a legacy code path, and so we don’t need to consider a backwards-incompatible 6.x release to remove the hack.

[gasman]

This fix is now released as django-modelcluster version 4.4.1 and 5.0.1 - the one remaining task we ought to do here is turn @johannesvogel’s example project into a test case on the Wagtail side, so that we can catch any future regressions in this setup.