Fix count for queries using Subquery or Exists

.travis.yml

@@ -55,4 +55,5 @@ install:
 
 script:
   - pip install -r django/tests/requirements/py3.txt
+  - python manage.py test
   - ./test.sh

manage.py

@@ -0,0 +1,10 @@
+#!/usr/bin/env python
+import os
+import sys
+
+if __name__ == "__main__":
+    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "testapp.settings")
+
+    from django.core.management import execute_from_command_line
+
+    execute_from_command_line(sys.argv)

sql_server/pyodbc/compiler.py

@@ -2,7 +2,7 @@
 from itertools import chain
 
 from django.db.models.aggregates import Avg, Count, StdDev, Variance
-from django.db.models.expressions import Exists, OrderBy, Ref, Value
+from django.db.models.expressions import Exists, OrderBy, Ref, Subquery, Value
 from django.db.models.functions import (
     Chr, ConcatPair, Greatest, Least, Length, LPad, Repeat, RPad, StrIndex, Substr, Trim
 )
@@ -374,6 +374,10 @@ def compile(self, node, select_format=False):
         node = self._as_microsoft(node)
         return super().compile(node, select_format)
 
+    def collapse_group_by(self, expressions, having):
+        expressions = super().collapse_group_by(expressions, having)
+        return [e for e in expressions if not isinstance(e, Subquery)]
+
     def _as_microsoft(self, node):
         as_microsoft = None
         if isinstance(node, Avg):

testapp/migrations/0001_initial.py

@@ -0,0 +1,31 @@
+# Generated by Django 2.2.8.dev20191112211527 on 2019-11-15 01:38
+
+from django.db import migrations, models
+import django
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = [
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Post',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('title', models.CharField(max_length=255, verbose_name='title')),
+            ],
+        ),
+        migrations.CreateModel(
+            name='Comment',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('post', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to='testapp.Post')),
+                ('text', models.TextField(verbose_name='text')),
+                ('created_at', models.DateTimeField(default=django.utils.timezone.now)),
+            ],
+        ),
+    ]

testapp/models.py

@@ -0,0 +1,18 @@
+from django.db import models
+from django.utils import timezone
+
+
+class Post(models.Model):
+    title = models.CharField('title', max_length=255)
+
+    def __str__(self):
+        return self.title
+
+
+class Comment(models.Model):
+    post = models.ForeignKey(Post, on_delete=models.CASCADE)
+    text = models.TextField('text')
+    created_at = models.DateTimeField(default=timezone.now)
+
+    def __str__(self):
+        return self.text

testapp/settings.py

@@ -5,6 +5,13 @@
     'other': dj_database_url.config(env='DATABASE_URL_OTHER', default='sqlite:///db.sqlite'),
 }
 
+INSTALLED_APPS = (
+    'django.contrib.contenttypes',
+    'django.contrib.staticfiles',
+    'django.contrib.auth',
+    'testapp',
+)
+
 SECRET_KEY = "django_tests_secret_key"
 
 # Use a fast hasher to speed up tests.

testapp/tests/test_expressions.py

@@ -0,0 +1,25 @@
+from django.db.models.expressions import Exists, OuterRef, Subquery
+from django.test import TestCase
+
+from ..models import Comment, Post
+
+
+class TestSubquery(TestCase):
+    def setUp(self):
+        self.post = Post.objects.create(title="foo")
+
+    def test_with_count(self):
+        newest = Comment.objects.filter(post=OuterRef('pk')).order_by('-created_at')
+        Post.objects.annotate(
+            post_exists=Subquery(newest.values('text')[:1])
+        ).filter(post_exists=True).count()
+
+
+class TestExists(TestCase):
+    def setUp(self):
+        self.post = Post.objects.create(title="foo")
+
+    def test_with_count(self):
+        Post.objects.annotate(
+            post_exists=Exists(Post.objects.all())
+        ).filter(post_exists=True).count()