Fixed #36426 -- Added support for further iterables in prefetch_related_objects().

Thanks Sarah Boyce for the review.

django/db/models/query.py

@@ -2333,8 +2333,8 @@ def normalize_prefetch_lookups(lookups, prefix=None):
 
 def prefetch_related_objects(model_instances, *related_lookups):
     """
-    Populate prefetched object caches for a list of model instances based on
-    the lookups/Prefetch instances given.
+    Populate prefetched object caches for an iterable of model instances based
+    on the lookups/Prefetch instances given.
     """
     if not model_instances:
         return  # nothing to do
@@ -2402,7 +2402,7 @@ def prefetch_related_objects(model_instances, *related_lookups):
             # We assume that objects retrieved are homogeneous (which is the
             # premise of prefetch_related), so what applies to first object
             # applies to all.
-            first_obj = obj_list[0]
+            first_obj = next(iter(obj_list))
             to_attr = lookup.get_current_to_attr(level)[0]
             prefetcher, descriptor, attr_found, is_fetched = get_prefetcher(
                 first_obj, through_attr, to_attr

docs/ref/models/querysets.txt

@@ -4223,8 +4223,9 @@ Prefetches the given lookups on an iterable of model instances. This is useful
 in code that receives a list of model instances as opposed to a ``QuerySet``;
 for example, when fetching models from a cache or instantiating them manually.
 
-Pass an iterable of model instances (must all be of the same class) and the
-lookups or :class:`Prefetch` objects you want to prefetch for. For example:
+Pass an iterable of model instances (must all be of the same class and able to
+be iterated multiple times) and the lookups or :class:`Prefetch` objects you
+want to prefetch for. For example:
 
 .. code-block:: pycon
 

tests/prefetch_related/test_prefetch_related_objects.py

@@ -1,3 +1,5 @@
+from collections import deque
+
 from django.db.models import Prefetch, prefetch_related_objects
 from django.test import TestCase
 
@@ -221,3 +223,32 @@ class PrefetchRelatedObjectsTests(TestCase):
 
         with self.assertNumQueries(0):
             self.assertCountEqual(book1.authors.all(), [self.author1, self.author2])
+
+    def test_prefetch_related_objects_with_various_iterables(self):
+        book = self.book1
+
+        class MyIterable:
+            def __iter__(self):
+                yield book
+
+        cases = {
+            "set": {book},
+            "tuple": (book,),
+            "dict_values": {"a": book}.values(),
+            "frozenset": frozenset([book]),
+            "deque": deque([book]),
+            "custom iterator": MyIterable(),
+        }
+        for case_type, case in cases.items():
+            with self.subTest(case=case_type):
+                # Clear the prefetch cache.
+                book._prefetched_objects_cache = {}
+                with self.assertNumQueries(1):
+                    prefetch_related_objects(case, "authors")
+                with self.assertNumQueries(0):
+                    self.assertCountEqual(
+                        book.authors.all(), [self.author1, self.author2, self.author3]
+                    )
+
+    def test_prefetch_related_objects_empty(self):
+        prefetch_related_objects([], "authors")