From 73c987eb3b6a96300f7238dff32caf53cafe2098 Mon Sep 17 00:00:00 2001
From: Samriddha9619 <sumitkumartripathi0@gmail.com>
Date: Mon, 8 Dec 2025 22:11:31 +0530
Subject: Fixed #36701 -- Fixed memory leak in ModelState.

---
 django/db/models/base.py          |  3 +++
 tests/model_regress/models.py     |  4 ++++
 tests/model_regress/test_state.py | 26 ++++++++++++++++++++++++++
 3 files changed, 33 insertions(+)

diff --git a/django/db/models/base.py b/django/db/models/base.py
index 8d7cbdd32d..ad3f0c5e23 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -494,6 +494,9 @@ class ModelState:
         state.pop("peers", None)
         return state
 
+    def __del__(self):
+        self.fields_cache.clear()
+
 
 class Model(AltersData, metaclass=ModelBase):
     def __init__(self, *args, **kwargs):
diff --git a/tests/model_regress/models.py b/tests/model_regress/models.py
index 350850393a..2af882f20f 100644
--- a/tests/model_regress/models.py
+++ b/tests/model_regress/models.py
@@ -45,6 +45,10 @@ class Worker(models.Model):
         return self.name
 
 
+class WorkerProfile(models.Model):
+    worker = models.OneToOneField(Worker, on_delete=models.CASCADE)
+
+
 class NonAutoPK(models.Model):
     name = models.CharField(max_length=10, primary_key=True)
 
diff --git a/tests/model_regress/test_state.py b/tests/model_regress/test_state.py
index 8b0f087829..8abcc58382 100644
--- a/tests/model_regress/test_state.py
+++ b/tests/model_regress/test_state.py
@@ -1,7 +1,33 @@
+import gc
+
 from django.db.models.base import ModelState, ModelStateFieldsCacheDescriptor
 from django.test import SimpleTestCase
+from django.test.utils import garbage_collect
+
+from .models import Worker, WorkerProfile
 
 
 class ModelStateTests(SimpleTestCase):
     def test_fields_cache_descriptor(self):
         self.assertIsInstance(ModelState.fields_cache, ModelStateFieldsCacheDescriptor)
+
+    def test_one_to_one_field_cycle_collection(self):
+        self.addCleanup(gc.set_debug, gc.get_debug())
+        gc.set_debug(gc.DEBUG_SAVEALL)
+
+        def clear_garbage():
+            del gc.garbage[:]
+
+        self.addCleanup(clear_garbage)
+
+        worker = Worker()
+        profile = WorkerProfile(worker=worker)
+        worker_id = id(worker)
+
+        del worker
+        del profile
+
+        garbage_collect()
+
+        leaked = [obj for obj in gc.garbage if id(obj) == worker_id]
+        self.assertEqual(leaked, [])
-- 
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