[3.0.x] Fixed #31863 -- Prevented mutating model state by copies of model instances.

Regression in bfb746f983aa741afa3709794e70f1e0ab6040b5.

Backport of 94ea79be137f3cb30949bf82198e96e094f2650d from master

2 files changed, 19 insertions, 1 deletions

diff --git a/django/db/models/base.py b/django/db/models/base.py
index 63801da3c2..814e6ee280 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -543,7 +543,10 @@ class Model(metaclass=ModelBase):
 
     def __getstate__(self):
         """Hook to allow choosing the attributes to pickle."""
-        return self.__dict__
+        state = self.__dict__.copy()
+        state['_state'] = copy.copy(state['_state'])
+        state['_state'].fields_cache = state['_state'].fields_cache.copy()
+        return state
 
     def __setstate__(self, state):
         msg = None
diff --git a/tests/model_regress/tests.py b/tests/model_regress/tests.py
index 28eed87008..87df240d81 100644
--- a/tests/model_regress/tests.py
+++ b/tests/model_regress/tests.py
@@ -1,3 +1,4 @@
+import copy
 import datetime
 from operator import attrgetter
 
@@ -256,3 +257,17 @@ class EvaluateMethodTest(TestCase):
         dept = Department.objects.create(pk=1, name='abc')
         dept.evaluate = 'abc'
         Worker.objects.filter(department=dept)
+
+
+class ModelFieldsCacheTest(TestCase):
+    def test_fields_cache_reset_on_copy(self):
+        department1 = Department.objects.create(id=1, name='department1')
+        department2 = Department.objects.create(id=2, name='department2')
+        worker1 = Worker.objects.create(name='worker', department=department1)
+        worker2 = copy.copy(worker1)
+
+        self.assertEqual(worker2.department, department1)
+        # Changing related fields doesn't mutate the base object.
+        worker2.department = department2
+        self.assertEqual(worker2.department, department2)
+        self.assertEqual(worker1.department, department1)