Fixed #29984 -- Added QuerySet.iterator() support for prefetching related objects.

Co-authored-by: Raphael Kimmig <raphael.kimmig@ampad.de>
Co-authored-by: Simon Charette <charette.s@gmail.com>

tests/prefetch_related/tests.py

@@ -7,7 +7,8 @@ from django.db.models import Prefetch, QuerySet, prefetch_related_objects
 from django.db.models.query import get_prefetcher
 from django.db.models.sql import Query
 from django.test import TestCase, override_settings
-from django.test.utils import CaptureQueriesContext
+from django.test.utils import CaptureQueriesContext, ignore_warnings
+from django.utils.deprecation import RemovedInDjango50Warning
 
 from .models import (
     Article, Author, Author2, AuthorAddress, AuthorWithAge, Bio, Book,
@@ -316,6 +317,38 @@ class PrefetchRelatedTests(TestDataMixin, TestCase):
             ['Anne', 'Charlotte', 'Emily', 'Jane'],
         )
 
+    def test_m2m_prefetching_iterator_with_chunks(self):
+        with self.assertNumQueries(3):
+            authors = [
+                b.authors.first()
+                for b in Book.objects.prefetch_related('authors').iterator(chunk_size=2)
+            ]
+        self.assertEqual(
+            authors,
+            [self.author1, self.author1, self.author3, self.author4],
+        )
+
+    @ignore_warnings(category=RemovedInDjango50Warning)
+    def test_m2m_prefetching_iterator_without_chunks(self):
+        # prefetch_related() is ignored.
+        with self.assertNumQueries(5):
+            authors = [
+                b.authors.first()
+                for b in Book.objects.prefetch_related('authors').iterator()
+            ]
+        self.assertEqual(
+            authors,
+            [self.author1, self.author1, self.author3, self.author4],
+        )
+
+    def test_m2m_prefetching_iterator_without_chunks_warning(self):
+        msg = (
+            'Using QuerySet.iterator() after prefetch_related() without '
+            'specifying chunk_size is deprecated.'
+        )
+        with self.assertWarnsMessage(RemovedInDjango50Warning, msg):
+            Book.objects.prefetch_related('authors').iterator()
+
 
 class RawQuerySetTests(TestDataMixin, TestCase):
     def test_basic(self):