[1.7.x] Refs #25693 -- Added a regression test for `to_attr` validation on forward m2m.

from django.core import exceptions

from django.db import connections, router, transaction, IntegrityError

from django.db.models.constants import LOOKUP_SEP

from django.db.models.fields import AutoField, Empty

from django.db.models.fields import AutoField, Empty, FieldDoesNotExist

from django.db.models.query_utils import (Q, select_related_descend,

    deferred_class_factory, InvalidQuery)

from django.db.models.deletion import Collector

        # We assume that objects retrieved are homogeneous (which is the premise

        # of prefetch_related), so what applies to first object applies to all.

        model = instances[0].__class__

        for related_m2m in model._meta.get_all_related_many_to_many_objects():

        opts = model._meta

        conflicts = False

        try:

            opts.get_field(to_attr)

        except FieldDoesNotExist:

            for related_m2m in opts.get_all_related_many_to_many_objects():

                if related_m2m.get_accessor_name() == to_attr:

                    conflicts = True

                    break

        else:

            conflicts = True

        if conflicts:

            msg = 'to_attr={} conflicts with a field on the {} model.'

            raise ValueError(msg.format(to_attr, model.__name__))

        self.assertTrue('prefetch_related' in str(cm.exception))

        self.assertTrue("name" in str(cm.exception))

    def test_m2m_shadow(self):

    def test_forward_m2m_to_attr_conflict(self):

        msg = 'to_attr=authors conflicts with a field on the Book model.'

        authors = Author.objects.all()

        with self.assertRaisesMessage(ValueError, msg):

            list(Book.objects.prefetch_related(

                Prefetch('authors', queryset=authors, to_attr='authors'),

            ))

        # Without the ValueError, an author was deleted due to the implicit

        # save of the relation assignment.

        self.assertEqual(self.book1.authors.count(), 3)

    def test_reverse_m2m_to_attr_conflict(self):

        msg = 'to_attr=books conflicts with a field on the Author model.'

        poems = Book.objects.filter(title='Poems')

        with self.assertRaisesMessage(ValueError, msg):