From 093fd479d6be1790c6dc174f9df3a895b50e8a2f Mon Sep 17 00:00:00 2001
From: Dima Kudosh <LagMegantic@yandex.ru>
Date: Sun, 25 Jun 2017 21:17:13 +0300
Subject: [PATCH] Fixed #28335 -- Allowed query expressions in Meta.ordering.

---
 django/db/models/base.py    |  4 ++--
 docs/ref/models/options.txt | 18 +++++++++++++++---
 docs/releases/2.0.txt       |  3 +++
 tests/ordering/models.py    |  6 ++++++
 tests/ordering/tests.py     | 12 +++++++++++-
 5 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/django/db/models/base.py b/django/db/models/base.py
index d39c0b9ea6..dd2ac1de8c 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -1553,8 +1553,8 @@ class Model(metaclass=ModelBase):
         errors = []
         fields = cls._meta.ordering
 
-        # Skip '?' fields.
-        fields = (f for f in fields if f != '?')
+        # Skip expressions and '?' fields.
+        fields = (f for f in fields if isinstance(f, str) and f != '?')
 
         # Convert "-field" to "field".
         fields = ((f[1:] if f.startswith('-') else f) for f in fields)
diff --git a/docs/ref/models/options.txt b/docs/ref/models/options.txt
index bf8cf10c33..33ff581d39 100644
--- a/docs/ref/models/options.txt
+++ b/docs/ref/models/options.txt
@@ -256,9 +256,10 @@ Django quotes column and table names behind the scenes.
 
         ordering = ['-order_date']
 
-    This is a tuple or list of strings. Each string is a field name with an optional
-    "-" prefix, which indicates descending order. Fields without a leading "-" will
-    be ordered ascending. Use the string "?" to order randomly.
+    This is a tuple or list of strings and/or query expressions. Each string is
+    a field name with an optional "-" prefix, which indicates descending order.
+    Fields without a leading "-" will be ordered ascending. Use the string "?"
+    to order randomly.
 
     For example, to order by a ``pub_date`` field ascending, use this::
 
@@ -272,9 +273,20 @@ Django quotes column and table names behind the scenes.
 
         ordering = ['-pub_date', 'author']
 
+    You can also use :doc:`query expressions </ref/models/expressions>`. To
+    order by ``author`` ascending and make null values sort last, use this::
+
+        from django.db.models import F
+
+        ordering = [F('author').asc(nulls_last=True)]
+
     Default ordering also affects :ref:`aggregation queries
     <aggregation-ordering-interaction>`.
 
+    .. versionchanged:: 2.0
+
+        Support for query expressions was added.
+
 .. warning::
 
     Ordering is not a free operation. Each field you add to the ordering
diff --git a/docs/releases/2.0.txt b/docs/releases/2.0.txt
index 4f92927c1e..a8530d11c2 100644
--- a/docs/releases/2.0.txt
+++ b/docs/releases/2.0.txt
@@ -283,6 +283,9 @@ Models
   different conditionals <conditional-aggregation>` to multiple aggregations
   over the same fields or relations.
 
+* Added support for expressions in :attr:`Meta.ordering
+  <django.db.models.Options.ordering>`.
+
 Requests and Responses
 ~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/tests/ordering/models.py b/tests/ordering/models.py
index d3024f5fc0..85e8c59bb6 100644
--- a/tests/ordering/models.py
+++ b/tests/ordering/models.py
@@ -42,6 +42,12 @@ class OrderedByAuthorArticle(Article):
         ordering = ('author', 'second_author')
 
 
+class OrderedByFArticle(Article):
+    class Meta:
+        proxy = True
+        ordering = (models.F('author').asc(nulls_first=True), 'id')
+
+
 class Reference(models.Model):
     article = models.ForeignKey(OrderedByAuthorArticle, models.CASCADE)
 
diff --git a/tests/ordering/tests.py b/tests/ordering/tests.py
index ff749331b9..dbc924b06b 100644
--- a/tests/ordering/tests.py
+++ b/tests/ordering/tests.py
@@ -5,7 +5,7 @@ from django.db.models import F
 from django.db.models.functions import Upper
 from django.test import TestCase
 
-from .models import Article, Author, Reference
+from .models import Article, Author, OrderedByFArticle, Reference
 
 
 class OrderingTests(TestCase):
@@ -368,3 +368,13 @@ class OrderingTests(TestCase):
         r1 = Reference.objects.create(article_id=self.a1.pk)
         r2 = Reference.objects.create(article_id=self.a2.pk)
         self.assertSequenceEqual(Reference.objects.all(), [r2, r1])
+
+    def test_default_ordering_by_f_expression(self):
+        """F expressions can be used in Meta.ordering."""
+        articles = OrderedByFArticle.objects.all()
+        articles.filter(headline='Article 2').update(author=self.author_2)
+        articles.filter(headline='Article 3').update(author=self.author_1)
+        self.assertQuerysetEqual(
+            articles, ['Article 1', 'Article 4', 'Article 3', 'Article 2'],
+            attrgetter('headline')
+        )