Prefetch related not working for parental many to many relations

👋

I want to apologize in advance, if I’m missing something obvious, but having the following example:

from django.db import models

from modelcluster.models import ClusterableModel
from modelcluster.fields import ParentalManyToManyField

class Movie(ClusterableModel):
   name = models.CharField(max_length=250)
   actors = ParentalManyToManyField('Actors', related_name='movies')


class Actors(models.Model):
   name = models.CharField(max_length=250)

When I do

Movie.objects.prefetch_related('actors')

I get the following error:

ValueError: 'actors' does not resolve to an item that supports prefetching - this is an invalid parameter to prefetch_related().

The question is - how am I supposed to do the prefetch_related on ParentalManyToManyField?

The context is Wagtail, but I’m posting this here, since the issues seems to be from the clusterable model.

Thanks!

[acarasimon96]

It seems that I’ve discovered the culprit of this nasty bug.

Following @RadoRado’s example above, I’ve noticed that swapping out ParentalManyToManyField to models.ManyToManyField allowed the use of prefetch_related in exchange of losing the ability to see the not-yet-saved Actors objects in Wagtail previews. But with some debugging using pdb, I found out that DeferringRelatedManager in fields.create_deferring_forward_many_to_many_manager is missing a get_prefetch_queryset function, which the Django ORM needs when attempting to reference a ParentalManyToManyField field through prefetch_related – in this case, Movie.actors. ManyRelatedManager (in which DeferringRelatedManager is based from I think) does have that missing function.

So you may be wondering at this point where in the Django source code does this check happen. It turns out that occurs in django.db.models.query.get_prefetcher (specifically line 1700 as of Django 2.2)

[harrislapiroff]

Ran into this today! Would be very excited to see @chosak’s PR cross the finish line.