Serializer handle_m2m_field() should honor any previous prefetch

[xx396]

Example:

from django.contrib.auth.models import Group
from django.core import serializers

groups = Group.objects.prefetch_related('permissions').all()
serializers.serialize('json', groups)

This will N+1 query the permissions as handle_m2m_field uses iterator() which bypasses any cache. Suggest serializers/python.py line 78 replaces iterator() with all()

[Tim Graham]

I guess it would be nice to fix that but removing iterator() negates the memory savings on the non-prefetch case, doesn't it?

[Claude Paroz]

I have two suggestion commits for possible fixes:

• ​https://github.com/claudep/django/commit/7173c7de
• ​https://github.com/claudep/django/commit/68bbb8ff

I can turn one of them into a PR if an ORM informed person could validate either approach.

[Simon Charette]

Claude, I think ​https://github.com/claudep/django/commit/7173c7de makes more sense as the API to retrieve cached many-to-many results should likely be added to RelatedManager (e.g. expose a get_prefetched_objects method) instead of QuerySet.iterator.

[Claude Paroz]

Thanks, ​PR.

[Mariusz Felisiak <felisiak.mariusz@…>]

In adf58311:

Fixed #29078 -- Made serializers respect prefetch_related() for m2m fields.