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

Thanks Julien Hartmann for the report.

Backport of 20d1cb33c2 from master

docs/ref/models/instances.txt

@@ -128,6 +128,18 @@ in the ``from_db()`` method.
 Refreshing objects from database
 ================================
 
+If you delete a field from a model instance, accessing it again reloads the
+value from the database::
+
+    >>> obj = MyModel.objects.first()
+    >>> del obj.field
+    >>> obj.field  # Loads the field from the database
+
+.. versionchanged:: 1.10
+
+    In older versions, accessing a deleted field raised ``AttributeError``
+    instead of reloading it.
+
 .. method:: Model.refresh_from_db(using=None, fields=None)
 
 If you need to reload a model's values from the database, you can use the

docs/releases/1.10.txt

@@ -884,6 +884,9 @@ Miscellaneous
 * The ``_base_manager`` and ``_default_manager`` attributes are removed from
   model instances. They remain accessible on the model class.
 
+* Accessing a deleted field on a model instance, e.g. after ``del obj.field``,
+  reloads the field's value instead of raising ``AttributeError``.
+
 .. _deprecated-features-1.10:
 
 Features deprecated in 1.10

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):