[1.10.x] Fixed #26787 -- Documented deleting and reloading of model instance fields.

Thanks Julien Hartmann for the report.

Backport of 20d1cb33c2 from master

tests/basic/tests.py

@@ -421,6 +421,19 @@ class ModelTest(TestCase):
             # hash)
             hash(Article())
 
+    def test_delete_and_access_field(self):
+        # Accessing a field after it's deleted from a model reloads its value.
+        pub_date = datetime.now()
+        article = Article.objects.create(headline='foo', pub_date=pub_date)
+        new_pub_date = article.pub_date + timedelta(days=10)
+        article.headline = 'bar'
+        article.pub_date = new_pub_date
+        del article.headline
+        with self.assertNumQueries(1):
+            self.assertEqual(article.headline, 'foo')
+        # Fields that weren't deleted aren't reloaded.
+        self.assertEqual(article.pub_date, new_pub_date)
+
 
 class ModelLookupTest(TestCase):
     def setUp(self):