Fixed #36701 -- Fixed memory leak in ModelState reference cycles.

Samriddha9619 · Samriddha9619 · commit 78c250496a8b · 2025-12-11T02:09:45.000+05:30
diff --git a/django/db/models/base.py b/django/db/models/base.py
@@ -494,6 +494,12 @@ def __getstate__(self):
         state.pop("peers", None)
         return state
 
+    def __del__(self):
+        try:
+            self.fields_cache.clear()
+        except AttributeError:
+            pass
+
 
 class Model(AltersData, metaclass=ModelBase):
     def __init__(self, *args, **kwargs):
diff --git a/tests/model_regress/models.py b/tests/model_regress/models.py
@@ -60,3 +60,11 @@ class Model2(models.Model):
 
 class Model3(models.Model):
     model2 = models.ForeignKey(Model2, models.CASCADE, unique=True, to_field="model1")
+
+
+class CycleParent(models.Model):
+    pass
+
+
+class CycleChild(models.Model):
+    parent = models.OneToOneField(CycleParent, on_delete=models.CASCADE)
diff --git a/tests/model_regress/test_state.py b/tests/model_regress/test_state.py
@@ -1,7 +1,31 @@
+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 CycleChild, CycleParent
 
 
 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 create_cycle():
+            p = CycleParent()
+            c = CycleChild(parent=p)
+            _ = c.parent
+            return id(p)
+
+        p_id = create_cycle()
+
+        garbage_collect()
+
+        leaked = [obj for obj in gc.garbage if id(obj) == p_id]
+        self.assertEqual(leaked, [])
+
+        del gc.garbage[:]
