Add a potential fix^W^Wterrible hack for EXISTS() subqueries

tests/testapp/test_queries.py

@@ -1,5 +1,3 @@
-from unittest import expectedFailure
-
 from django import forms
 from django.core.exceptions import ValidationError
 from django.db import connections, models
@@ -479,16 +477,8 @@ def test_reference(self):
             [references.child2_1, references.child2_2],
         )
 
-    def test_reference_isnull(self):
-        self.assertSequenceEqual(
-            Model.objects.with_tree_fields().filter(referencemodel__isnull=True), []
-        )
-        self.assertSequenceEqual(
-            Model.objects.with_tree_fields().exclude(referencemodel__isnull=True), []
-        )
-
-    @expectedFailure
     def test_reference_isnull_issue63(self):
+        # https://github.com/feincms/django-tree-queries/issues/63
         self.assertSequenceEqual(
             Model.objects.with_tree_fields().exclude(referencemodel__isnull=False), []
         )

tree_queries/compiler.py

@@ -186,6 +186,11 @@ def get_sibling_order_params(self):
         return ordering_params
 
     def as_sql(self, *args, **kwargs):
+        # Try detecting if we're used in a EXISTS() subquery; we do not need
+        # the tree table in that case. See GitHub issue #63.
+        if self.query.subquery and self.query.annotation_select_mask == ["a"]:
+            return super().as_sql(*args, **kwargs)
+
         # The general idea is that if we have a summary query (e.g. .count())
         # then we do not want to ask Django to add the tree fields to the query
         # using .query.add_extra. The way to determine whether we have a