[2.0.x] Fixed #29195 -- Fixed Exists.output_field resolution on single-valued queries.

The Subquery class which Exists inherits from defaulted to using single-valued
querie's field if no output_field was explicitly specified on initialization
which was bypassing the Exists.output_field defined at the class level.

Moving Subquery's dynamic output_field resolution to _resolve_output_field
should make sure the fallback logic is only performed if required.

Regression in 08654a99.

Thanks Oli Warner for the detailed report.

Backport of 277ed072 from master

django/db/models/expressions.py

@@ -972,10 +972,13 @@ class Subquery(Expression):
     def __init__(self, queryset, output_field=None, **extra):
         self.queryset = queryset
         self.extra = extra
-        if output_field is None and len(self.queryset.query.select) == 1:
-            output_field = self.queryset.query.select[0].field
         super().__init__(output_field)
 
+    def _resolve_output_field(self):
+        if len(self.queryset.query.select) == 1:
+            return self.queryset.query.select[0].field
+        return super()._resolve_output_field()
+
     def copy(self):
         clone = super().copy()
         clone.queryset = clone.queryset.all()

docs/releases/2.0.4.txt

@@ -9,4 +9,5 @@ Django 2.0.4 fixes several bugs in 2.0.3.
 Bugfixes
 ========
 
-* ...
+* Fixed a crash when filtering with an ``Exists()`` annotation of a queryset
+  containing a single field (:ticket:`29195`).

tests/expressions/tests.py

@@ -417,6 +417,10 @@ class BasicExpressionsTests(TestCase):
         outer = Employee.objects.annotate(is_point_of_contact=Exists(inner))
         self.assertIs(outer.exists(), True)
 
+    def test_exist_single_field_output_field(self):
+        queryset = Company.objects.values('pk')
+        self.assertIsInstance(Exists(queryset).output_field, models.BooleanField)
+
     def test_subquery(self):
         Company.objects.filter(name='Example Inc.').update(
             point_of_contact=Employee.objects.get(firstname='Joe', lastname='Smith'),