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>

django/db/models/query.py

@@ -5,7 +5,7 @@ The main QuerySet implementation. This provides the public API for the ORM.
 import copy
 import operator
 import warnings
-from itertools import chain
+from itertools import chain, islice
 
 import django
 from django.conf import settings
@@ -23,6 +23,7 @@ from django.db.models.query_utils import FilteredRelation, Q
 from django.db.models.sql.constants import CURSOR, GET_ITERATOR_CHUNK_SIZE
 from django.db.models.utils import create_namedtuple_class, resolve_callables
 from django.utils import timezone
+from django.utils.deprecation import RemovedInDjango50Warning
 from django.utils.functional import cached_property, partition
 
 # The maximum number of results to fetch in a get() query.
@@ -356,14 +357,40 @@ class QuerySet:
     ####################################
 
     def _iterator(self, use_chunked_fetch, chunk_size):
-        yield from self._iterable_class(self, chunked_fetch=use_chunked_fetch, chunk_size=chunk_size)
+        iterable = self._iterable_class(
+            self,
+            chunked_fetch=use_chunked_fetch,
+            chunk_size=chunk_size or 2000,
+        )
+        if not self._prefetch_related_lookups or chunk_size is None:
+            yield from iterable
+            return
 
-    def iterator(self, chunk_size=2000):
+        iterator = iter(iterable)
+        while results := list(islice(iterator, chunk_size)):
+            prefetch_related_objects(results, *self._prefetch_related_lookups)
+            yield from results
+
+    def iterator(self, chunk_size=None):
         """
         An iterator over the results from applying this QuerySet to the
-        database.
+        database. chunk_size must be provided for QuerySets that prefetch
+        related objects. Otherwise, a default chunk_size of 2000 is supplied.
         """
-        if chunk_size <= 0:
+        if chunk_size is None:
+            if self._prefetch_related_lookups:
+                # When the deprecation ends, replace with:
+                # raise ValueError(
+                #     'chunk_size must be provided when using '
+                #     'QuerySet.iterator() after prefetch_related().'
+                # )
+                warnings.warn(
+                    'Using QuerySet.iterator() after prefetch_related() '
+                    'without specifying chunk_size is deprecated.',
+                    category=RemovedInDjango50Warning,
+                    stacklevel=2,
+                )
+        elif chunk_size <= 0:
             raise ValueError('Chunk size must be strictly positive.')
         use_chunked_fetch = not connections[self.db].settings_dict.get('DISABLE_SERVER_SIDE_CURSORS')
         return self._iterator(use_chunked_fetch, chunk_size)