Can't add ManytoMany fields to a Page

For example

class BlogPage(Page):
    body = RichTextField()
    categories = models.ManyToManyField(BlogCategory, blank=True)

One gets ValueError “<BlogPage: foo>” needs to have a value for field “blogpage” before this many-to-many relationship can be used.

It’s caused by this https://github.com/torchbox/django-modelcluster/issues/3 https://github.com/torchbox/django-modelcluster/blob/master/modelcluster/forms.py#L260

I’ll keep trying to think of a solution. Seems like a common use case as Wordpress implements categories as a M2M (you can pick multiple, but they are predefined unlike a tag).

[gasman]

This is now implemented in django-modelcluster: https://github.com/torchbox/django-modelcluster/pull/62

From initial tests, it seems to be working nicely with Wagtail: just use ParentalManyToManyField instead of ManyToManyField, and specify it in a FieldPanel as normal.

Things that need to be done before the release of Wagtail 1.9:

• release django-modelcluster 3.0 final and update wagtail’s setup.py to require django-modelcluster>=3.0,<4.0
• add a release note to document the updated django-modelcluster / django-taggit dependencies (since it’s somewhat likely that people will have the old versions hard-coded in their requirements files)
• ensure that M2M relations are covered by tests
• document the use of ParentalManyToManyField, possibly through a new ‘categories’ section in the tutorial (which was dropped from https://github.com/wagtail/wagtail/pull/3125#issuecomment-259181336 because it was too messy to implement without real M2M support) as well as http://docs.wagtail.io/en/v1.8/topics/pages.html

[mattots]

I’ve just come up against exactly the scenario bufke raised in the initial comment - the need to provide an interface for adding categories to blog posts. The workaround using an inline is not very pretty, and tags don’t quite fulfil this particular use case. Is there really no proper M2M support in Wagtail? If not, can I add a massive +1 for this to be addressed please.