Fix Exists in order by (#30)

Author: OskarPersson

sql_server/pyodbc/compiler.py

@@ -3,7 +3,7 @@
 
 import django
 from django.db.models.aggregates import Avg, Count, StdDev, Variance
-from django.db.models.expressions import OrderBy, Ref, Subquery, Value
+from django.db.models.expressions import Ref, Subquery, Value
 from django.db.models.functions import (
     Chr, ConcatPair, Greatest, Least, Length, LPad, Repeat, RPad, StrIndex, Substr, Trim
 )
@@ -70,15 +70,6 @@ def _as_sql_lpad(self, compiler, connection):
     return template % {'expression': expression, 'length': length, 'fill_text': fill_text}, params
 
 
-def _as_sql_order_by(self, compiler, connection):
-    template = None
-    if self.nulls_last:
-        template = 'CASE WHEN %(expression)s IS NULL THEN 1 ELSE 0 END, %(expression)s %(ordering)s'
-    if self.nulls_first:
-        template = 'CASE WHEN %(expression)s IS NULL THEN 0 ELSE 1 END, %(expression)s %(ordering)s'
-    return self.as_sql(compiler, connection, template=template)
-
-
 def _as_sql_repeat(self, compiler, connection):
     return self.as_sql(compiler, connection, function='REPLICATE')
 
@@ -390,8 +381,6 @@ def _as_microsoft(self, node):
             as_microsoft = _as_sql_rpad
         elif isinstance(node, LPad):
             as_microsoft = _as_sql_lpad
-        elif isinstance(node, OrderBy):
-            as_microsoft = _as_sql_order_by
         elif isinstance(node, Repeat):
             as_microsoft = _as_sql_repeat
         elif isinstance(node, StdDev):

sql_server/pyodbc/functions.py

@@ -2,7 +2,7 @@
 from django.db.models import BooleanField
 from django.db.models.functions import Cast
 from django.db.models.functions.math import ATan2, Log, Ln, Round
-from django.db.models.expressions import Case, Exists, When
+from django.db.models.expressions import Case, Exists, OrderBy, When
 from django.db.models.lookups import Lookup
 
 DJANGO3 = VERSION[0] >= 3
@@ -53,6 +53,27 @@ def sqlserver_lookup(self, compiler, connection):
     return lookup.as_sql(compiler, connection)
 
 
+def sqlserver_orderby(self, compiler, connection):
+    # MSSQL doesn't allow ORDER BY EXISTS() unless it's wrapped in
+    # a CASE WHEN.
+
+    template = None
+    if self.nulls_last:
+        template = 'CASE WHEN %(expression)s IS NULL THEN 1 ELSE 0 END, %(expression)s %(ordering)s'
+    if self.nulls_first:
+        template = 'CASE WHEN %(expression)s IS NULL THEN 0 ELSE 1 END, %(expression)s %(ordering)s'
+
+    if isinstance(self.expression, Exists):
+        copy = self.copy()
+        copy.expression = Case(
+            When(self.expression, then=True),
+            default=False,
+            output_field=BooleanField(),
+        )
+        return copy.as_sql(compiler, connection, template=template)
+    return self.as_sql(compiler, connection, template=template)
+
+
 ATan2.as_microsoft = sqlserver_atan2
 Log.as_microsoft = sqlserver_log
 Ln.as_microsoft = sqlserver_ln
@@ -62,3 +83,5 @@ def sqlserver_lookup(self, compiler, connection):
     Lookup.as_microsoft = sqlserver_lookup
 else:
     Exists.as_microsoft = sqlserver_exists
+
+OrderBy.as_microsoft = sqlserver_orderby

testapp/tests/test_expressions.py

@@ -44,6 +44,15 @@ def test_with_case_when(self):
         ).get()
         self.assertEqual(author.has_post, 1)
 
+    @skipUnless(DJANGO3, "Django 3 specific tests")
+    def test_order_by_exists(self):
+        author_without_posts = Author.objects.create(name="other author")
+        authors_by_posts = Author.objects.order_by(Exists(Post.objects.filter(author=OuterRef('pk'))).desc())
+        self.assertSequenceEqual(authors_by_posts, [self.author, author_without_posts])
+
+        authors_by_posts = Author.objects.order_by(Exists(Post.objects.filter(author=OuterRef('pk'))).asc())
+        self.assertSequenceEqual(authors_by_posts, [author_without_posts, self.author])
+
 
 @skipUnlessDBFeature('supports_partially_nullable_unique_constraints')
 class TestPartiallyNullableUniqueTogether(TestCase):