[5.2.x] Fixed #36416 -- Made QuerySet.in_bulk() account for composite pks in id_list.

Backport of 26313bc21932d0d3af278ab387549d63b1f64575 from main.

3 files changed, 23 insertions, 1 deletions

diff --git a/django/db/models/query.py b/django/db/models/query.py
index bb5462037c..1270141619 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -1149,7 +1149,9 @@ class QuerySet(AltersData):
             if not id_list:
                 return {}
             filter_key = "{}__in".format(field_name)
-            batch_size = connections[self.db].features.max_query_params
+            max_params = connections[self.db].features.max_query_params or 0
+            num_fields = len(opts.pk_fields) if field_name == "pk" else 1
+            batch_size = max_params // num_fields
             id_list = tuple(id_list)
             # If the database has a limit on the number of query parameters
             # (e.g. SQLite), retrieve objects in batches if necessary.
diff --git a/docs/releases/5.2.2.txt b/docs/releases/5.2.2.txt
index 5647c0cc07..3870a3efa4 100644
--- a/docs/releases/5.2.2.txt
+++ b/docs/releases/5.2.2.txt
@@ -34,3 +34,7 @@ Bugfixes
 * Fixed a regression in Django 5.2 where admin's ``filter_horizontal`` buttons
   lacked ``type="button"``, causing them to intercept form submission when
   pressing the Enter key (:ticket:`36423`).
+
+* Fixed a bug in Django 5.2 where calling ``QuerySet.in_bulk()`` with an
+  ``id_list`` argument on models with a ``CompositePrimaryKey`` failed to
+  observe database parameter limits (:ticket:`36416`).
diff --git a/tests/composite_pk/tests.py b/tests/composite_pk/tests.py
index 1a28a585fa..2245a472e4 100644
--- a/tests/composite_pk/tests.py
+++ b/tests/composite_pk/tests.py
@@ -147,6 +147,22 @@ class CompositePKTests(TestCase):
         result = Comment.objects.in_bulk([self.comment.pk])
         self.assertEqual(result, {self.comment.pk: self.comment})
 
+    @unittest.mock.patch.object(
+        type(connection.features), "max_query_params", new_callable=lambda: 10
+    )
+    def test_in_bulk_batching(self, mocked_max_query_params):
+        Comment.objects.all().delete()
+        num_requiring_batching = (connection.features.max_query_params // 2) + 1
+        comments = [
+            Comment(id=i, tenant=self.tenant, user=self.user)
+            for i in range(1, num_requiring_batching + 1)
+        ]
+        Comment.objects.bulk_create(comments)
+        id_list = list(Comment.objects.values_list("pk", flat=True))
+        with self.assertNumQueries(2):
+            comment_dict = Comment.objects.in_bulk(id_list=id_list)
+        self.assertQuerySetEqual(comment_dict, id_list)
+
     def test_iterator(self):
         """
         Test the .iterator() method of composite_pk models.