[5.1.x] Fixed #36197 -- Fixed improper many-to-many count() and exists() for non-pk to_field.

Regression in 66e47ac69a7e71cf32eee312d05668d8f1ba24bb.

Thanks mfontana-elem for the report and Sarah for the tests.

Backport of c3a23aa02faa1cf1d32e43d66858e793cd9ecac4 from main.

3 files changed, 14 insertions, 1 deletions

diff --git a/django/db/models/fields/related_descriptors.py b/django/db/models/fields/related_descriptors.py
index bc288c47ec..6541df1b7c 100644
--- a/django/db/models/fields/related_descriptors.py
+++ b/django/db/models/fields/related_descriptors.py
@@ -1219,7 +1219,7 @@ def create_forward_many_to_many_manager(superclass, rel, reverse):
                 return None
             hints = {"instance": self.instance}
             manager = self.through._base_manager.db_manager(db, hints=hints)
-            filters = {self.source_field_name: self.instance.pk}
+            filters = {self.source_field_name: self.related_val[0]}
             # Nullable target rows must be excluded as well as they would have
             # been filtered out from an INNER JOIN.
             if self.target_field.null:
diff --git a/docs/releases/5.1.7.txt b/docs/releases/5.1.7.txt
index deda4f2f92..c32f8f5ae1 100644
--- a/docs/releases/5.1.7.txt
+++ b/docs/releases/5.1.7.txt
@@ -16,3 +16,8 @@ Bugfixes
 * Fixed a bug in Django 5.1 where ``FileSystemStorage``, with
   ``allow_overwrite`` set to ``True``, did not truncate the overwritten file
   content (:ticket:`36191`).
+
+* Fixed a regression in Django 5.1 where the ``count`` and ``exists`` methods
+  of ``ManyToManyField`` related managers would always return ``0`` and
+  ``False`` when the intermediary model back references used ``to_field``
+  (:ticket:`36197`).
diff --git a/tests/m2m_through/tests.py b/tests/m2m_through/tests.py
index 83449a7c7b..81a47a2083 100644
--- a/tests/m2m_through/tests.py
+++ b/tests/m2m_through/tests.py
@@ -533,3 +533,11 @@ class M2mThroughToFieldsTests(TestCase):
             [choice[0] for choice in field.get_choices(include_blank=False)],
             ["pea", "potato", "tomato"],
         )
+
+    def test_count(self):
+        self.assertEqual(self.curry.ingredients.count(), 3)
+        self.assertEqual(self.tomato.recipes.count(), 1)
+
+    def test_exists(self):
+        self.assertTrue(self.curry.ingredients.exists())
+        self.assertTrue(self.tomato.recipes.exists())