prefetch_related makes me sad

gistfile1.txt

In : from onefinestay.geo.models import Place
In : p = Place.objects.prefetch_related('categories').get(id=1460)  # Only prefetching categories

In : p.categories.all()
Out: [<Category: Miscellaneous>, <Category: For kids>]

In : p.regions.all()
Out: [<Region: Belgravia>, <Region: Bloomsbury>, <Region: Clerkenwell>, <Region: City of London>]

In : p.regions.clear()

In : p.regions.all()
Out: []

In : p.categories.clear()

In : p.categories.all()
Out: [<Category: Miscellaneous>, <Category: For kids>]  # <---- Booooooooo

Did you do any investigation on this? What is that p.categories.clear() is really doing?

The same as p.regions.clear(). It's deleting all the place->category m2m models:

https://github.com/django/django/blob/master/django/db/models/fields/related.py#L708-L725

I'm pretty sure it's not working because the prefetch_related code isn't smart enough to detect when the .all() queryset would have actually changed.