Fix Exists in Case(When) (#29)

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 Exists, OrderBy, Ref, Subquery, Value
+from django.db.models.expressions import OrderBy, 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_exists(self, compiler, connection, template=None, **extra_context):
-    # MS SQL doesn't allow EXISTS() in the SELECT list, so wrap it with a
-    # CASE WHEN expression. Change the template since the When expression
-    # requires a left hand side (column) to compare against.
-    sql, params = self.as_sql(compiler, connection, template, **extra_context)
-    sql = 'CASE WHEN {} THEN 1 ELSE 0 END'.format(sql)
-    return sql, params
-
-
 def _as_sql_order_by(self, compiler, connection):
     template = None
     if self.nulls_last:
@@ -399,8 +390,6 @@ def _as_microsoft(self, node):
             as_microsoft = _as_sql_rpad
         elif isinstance(node, LPad):
             as_microsoft = _as_sql_lpad
-        elif isinstance(node, Exists):
-            as_microsoft = _as_sql_exists
         elif isinstance(node, OrderBy):
             as_microsoft = _as_sql_order_by
         elif isinstance(node, Repeat):

sql_server/pyodbc/features.py

@@ -21,6 +21,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     ignores_quoted_identifier_case = True
     requires_literal_defaults = True
     requires_sqlparse_for_splitting = False
+    supports_boolean_expr_in_select_clause = False
     supports_ignore_conflicts = False
     supports_index_on_text_field = False
     supports_paramstyle_pyformat = False

sql_server/pyodbc/functions.py

@@ -1,5 +1,11 @@
+from django import VERSION
+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.lookups import Lookup
+
+DJANGO3 = VERSION[0] >= 3
 
 
 class TryCast(Cast):
@@ -24,7 +30,35 @@ def sqlserver_round(self, compiler, connection, **extra_context):
     return self.as_sql(compiler, connection, template='%(function)s(%(expressions)s, 0)', **extra_context)
 
 
+def sqlserver_exists(self, compiler, connection, template=None, **extra_context):
+    # MS SQL doesn't allow EXISTS() in the SELECT list, so wrap it with a
+    # CASE WHEN expression. Change the template since the When expression
+    # requires a left hand side (column) to compare against.
+    sql, params = self.as_sql(compiler, connection, template, **extra_context)
+    sql = 'CASE WHEN {} THEN 1 ELSE 0 END'.format(sql)
+    return sql, params
+
+
+def sqlserver_lookup(self, compiler, connection):
+    # MSSQL doesn't allow EXISTS() to be compared to another expression
+    # unless it's wrapped in a CASE WHEN.
+    wrapped = False
+    exprs = []
+    for expr in (self.lhs, self.rhs):
+        if isinstance(expr, Exists):
+            expr = Case(When(expr, then=True), default=False, output_field=BooleanField())
+            wrapped = True
+        exprs.append(expr)
+    lookup = type(self)(*exprs) if wrapped else self
+    return lookup.as_sql(compiler, connection)
+
+
 ATan2.as_microsoft = sqlserver_atan2
 Log.as_microsoft = sqlserver_log
 Ln.as_microsoft = sqlserver_ln
 Round.as_microsoft = sqlserver_round
+
+if DJANGO3:
+    Lookup.as_microsoft = sqlserver_lookup
+else:
+    Exists.as_microsoft = sqlserver_exists

testapp/tests/test_expressions.py

@@ -1,9 +1,15 @@
-from django.db.models.expressions import Exists, OuterRef, Subquery
+from unittest import skipUnless
+
+from django import VERSION
+from django.db.models import IntegerField
+from django.db.models.expressions import Case, Exists, OuterRef, Subquery, Value, When
 from django.db.utils import IntegrityError
 from django.test import TestCase, skipUnlessDBFeature
 
 from ..models import Author, Comment, Editor, Post
 
+DJANGO3 = VERSION[0] >= 3
+
 
 class TestSubquery(TestCase):
     def setUp(self):
@@ -27,6 +33,17 @@ def test_with_count(self):
             post_exists=Exists(Post.objects.all())
         ).filter(post_exists=True).count()
 
+    @skipUnless(DJANGO3, "Django 3 specific tests")
+    def test_with_case_when(self):
+        author = Author.objects.annotate(
+            has_post=Case(
+                When(Exists(Post.objects.filter(author=OuterRef('pk')).values('pk')), then=Value(1)),
+                default=Value(0),
+                output_field=IntegerField(),
+            )
+        ).get()
+        self.assertEqual(author.has_post, 1)
+
 
 @skipUnlessDBFeature('supports_partially_nullable_unique_constraints')
 class TestPartiallyNullableUniqueTogether(TestCase):