Django, Wagtail admin: handle many to many with `through` - django

Question

I have 2 models with a through table such as:
class A(models.Model):
title = models.CharField(max_length=500)
bs = models.ManyToManyField(to='app.B', through='app.AB', blank=True)
content_panels = [
FieldPanel('title'),
FieldPanel('fields'), # what should go here?
]
class AB(models.Model):
a = models.ForeignKey(to='app.A')
b = models.ForeignKey(to='app.B')
position = models.IntegerField()
class Meta:
unique_together = ['a', 'b']
I'm getting the following error when trying to save:
Cannot set values on a ManyToManyField which specifies an intermediary model.
That error makes sense to me. I should save AB instances instead. I'm just unsure what's the best way to achieve that in Wagtail.

Answer 1

What you need is an InlinePanel: http://docs.wagtail.io/en/v2.0.1/getting_started/tutorial.html#images
from wagtail.admin.edit_handlers import FieldPanel, InlinePanel
from wagtail.core.models import Orderable
from modelcluster.fields import ParentalKey
# the parent object must inherit from ClusterableModel to allow parental keys;
# this happens automatically for Page models
from modelcluster.models import ClusterableModel
class A(ClusterableModel):
title = models.CharField(max_length=500)
content_panels = [
FieldPanel('title'),
InlinePanel('ab_objects'),
]
class AB(Orderable):
a = ParentalKey('app.A', related_name='ab_objects')
b = models.ForeignKey('app.B')
panels = [
FieldPanel('b'),
]