Fixed #13312 -- Added a way to customize the order_by() of null fields

django/db/models/expressions.py

@@ -462,11 +462,11 @@ def __repr__(self):
     def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
         return query.resolve_ref(self.name, allow_joins, reuse, summarize)
 
-    def asc(self):
-        return OrderBy(self)
+    def asc(self, **kwargs):
+        return OrderBy(self, **kwargs)
 
-    def desc(self):
-        return OrderBy(self, descending=True)
+    def desc(self, **kwargs):
+        return OrderBy(self, descending=True, **kwargs)
 
 
 class Func(Expression):
@@ -862,7 +862,12 @@ def as_sql(self, compiler, connection, template=None, case_joiner=None, **extra_
 class OrderBy(BaseExpression):
     template = '%(expression)s %(ordering)s'
 
-    def __init__(self, expression, descending=False):
+    def __init__(self, expression, descending=False, nulls_first=False,
+                 nulls_last=False):
+        if nulls_first and nulls_last:
+            raise ValueError("nulls_first and nulls_last are mutualy exclusive")
+        self.nulls_first = nulls_first
+        self.nulls_last = nulls_last
         self.descending = descending
         if not hasattr(expression, 'resolve_expression'):
             raise ValueError('expression must be an expression type')
@@ -879,6 +884,11 @@ def get_source_expressions(self):
         return [self.expression]
 
     def as_sql(self, compiler, connection, template=None, **extra_context):
+        if not template:
+            if self.nulls_last:
+                template = "%s NULLS LAST" % self.template
+            elif self.nulls_first:
+                template = "%s NULLS FIRST" % self.template
         connection.ops.check_expression_support(self)
         expression_sql, params = compiler.compile(self.expression)
         placeholders = {
@@ -889,6 +899,22 @@ def as_sql(self, compiler, connection, template=None, **extra_context):
         template = template or self.template
         return (template % placeholders).rstrip(), params
 
+    def as_sqlite(self, compiler, connection):
+        template = None
+        if self.nulls_last:
+            template = '%(expression)s IS NULL, %(expression)s %(ordering)s'
+        elif self.nulls_first:
+            template = '%(expression)s IS NOT NULL, %(expression)s %(ordering)s'
+        return self.as_sql(compiler, connection, template=template)
+
+    def as_mysql(self, compiler, connection):
+        template = None
+        if self.nulls_last:
+            template = 'IF(ISNULL(%(expression)s),1,0),%(expression)s %(ordering)s '
+        elif self.nulls_first:
+            template = 'IF(ISNULL(%(expression)s),0,1),%(expression)s %(ordering)s '
+        return self.as_sql(compiler, connection, template=template)
+
     def get_group_by_cols(self):
         cols = []
         for source in self.get_source_expressions():

tests/ordering/tests.py

@@ -10,19 +10,25 @@
 
 
 class OrderingTests(TestCase):
-    def setUp(self):
-        self.a1 = Article.objects.create(
+
+    @classmethod
+    def setUpTestData(cls):
+        cls.a1 = Article.objects.create(
             headline="Article 1", pub_date=datetime(2005, 7, 26)
         )
-        self.a2 = Article.objects.create(
+        cls.a2 = Article.objects.create(
             headline="Article 2", pub_date=datetime(2005, 7, 27)
         )
-        self.a3 = Article.objects.create(
+        cls.a3 = Article.objects.create(
             headline="Article 3", pub_date=datetime(2005, 7, 27)
         )
-        self.a4 = Article.objects.create(
+        cls.a4 = Article.objects.create(
             headline="Article 4", pub_date=datetime(2005, 7, 28)
         )
+        cls.author_1 = Author.objects.create()
+        cls.author_2 = Author.objects.create()
+        for i in range(3):
+            Author.objects.create()
 
     def test_default_ordering(self):
         """
@@ -90,6 +96,58 @@ def test_order_by_override(self):
             attrgetter("headline")
         )
 
+    def test_order_by_nulls_first_and_last(self):
+        with self.assertRaisesMessage(ValueError,
+                                      "nulls_first and nulls_last are mutualy exclusive"):
+            Article.objects.order_by(F("author").desc(nulls_last=True, nulls_first=True))
+
+    def test_order_by_nulls_last(self):
+        Article.objects.filter(headline="Article 3").update(author=self.author_1)
+        Article.objects.filter(headline="Article 4").update(author=self.author_2)
+
+        # check that asc and desc are chainable with nulls_last
+
+        self.assertQuerysetEqual(
+            Article.objects.order_by(F("author").desc(nulls_last=True)), [
+                "Article 4",
+                "Article 3",
+                "Article 1",
+                "Article 2",
+            ],
+            attrgetter("headline")
+        )
+
+        self.assertQuerysetEqual(
+            Article.objects.order_by(F("author").asc(nulls_last=True)), [
+                "Article 3",
+                "Article 4",
+                "Article 1",
+                "Article 2",
+            ],
+            attrgetter("headline")
+        )
+
+    def test_order_by_nulls_first(self):
+        self.assertQuerysetEqual(
+            Article.objects.order_by(F("author").asc(nulls_first=True)), [
+                "Article 1",
+                "Article 2",
+                "Article 3",
+                "Article 4",
+            ],
+            attrgetter("headline")
+        )
+
+        self.assertQuerysetEqual(
+            Article.objects.order_by(F("author").desc(nulls_first=True)), [
+                "Article 1",
+                "Article 2",
+                "Article 3",
+                "Article 4",
+            ],
+            attrgetter("headline")
+        )
+
     def test_stop_slicing(self):
         """
         Use the 'stop' part of slicing notation to limit the results.
@@ -213,14 +271,10 @@ def test_order_by_pk(self):
         Ensure that 'pk' works as an ordering option in Meta.
         Refs #8291.
         """
-        Author.objects.create(pk=1)
-        Author.objects.create(pk=2)
-        Author.objects.create(pk=3)
-        Author.objects.create(pk=4)
 
         self.assertQuerysetEqual(
             Author.objects.all(), [
-                4, 3, 2, 1
+                5, 4, 3, 2, 1
             ],
             attrgetter("pk")
         )
@@ -232,7 +286,7 @@ def test_order_by_fk_attname(self):
         Refs #19195.
         """
         for i in range(1, 5):
-            author = Author.objects.create(pk=i)
+            author = Author.objects.get(pk=i)
             article = getattr(self, "a%d" % (5 - i))
             article.author = author
             article.save(update_fields={'author'})