Fixed #28199 -- Fixed Subquery generating unnecessary/invalid CAST.

Thanks Simon Charette for the fix.

django/db/models/expressions.py

@@ -984,7 +984,6 @@ class Subquery(Expression):
 
         template = template or template_params.get('template', self.template)
         sql = template % template_params
-        sql = connection.ops.unification_cast_sql(self.output_field) % sql
         return sql, sql_params
 
     def _prepare(self, output_field):

docs/releases/1.11.2.txt

@@ -39,3 +39,6 @@ Bugfixes
 
 * Fixed ``MultipleObjectMixin.paginate_queryset()`` crash on Python 2 if the
   ``InvalidPage`` message contains non-ASCII (:ticket:`28204`).
+
+* Prevented ``Subquery`` from adding an unnecessary ``CAST`` which resulted in
+  invalid SQL (:ticket:`28199`).

tests/expressions/models.py

@@ -1,6 +1,7 @@
 """
 Tests for F() query expression syntax.
 """
+import uuid
 
 from django.db import models
 
@@ -79,8 +80,13 @@ class SimulationRun(models.Model):
         return "%s (%s to %s)" % (self.midpoint, self.start, self.end)
 
 
+class UUIDPK(models.Model):
+    id = models.UUIDField(primary_key=True, default=uuid.uuid4)
+
+
 class UUID(models.Model):
     uuid = models.UUIDField(null=True)
+    uuid_fk = models.ForeignKey(UUIDPK, models.CASCADE, null=True)
 
     def __str__(self):
         return "%s" % self.uuid

tests/expressions/tests.py

@@ -24,7 +24,8 @@ from django.test import (
 from django.test.utils import Approximate
 
 from .models import (
-    UUID, Company, Employee, Experiment, Number, Result, SimulationRun, Time,
+    UUID, UUIDPK, Company, Employee, Experiment, Number, Result, SimulationRun,
+    Time,
 )
 
 
@@ -479,6 +480,12 @@ class BasicExpressionsTests(TestCase):
         subquery_test2 = Company.objects.filter(pk=Subquery(small_companies.filter(num_employees=3)))
         self.assertCountEqual(subquery_test2, [self.foobar_ltd])
 
+    def test_uuid_pk_subquery(self):
+        u = UUIDPK.objects.create()
+        UUID.objects.create(uuid_fk=u)
+        qs = UUIDPK.objects.filter(id__in=Subquery(UUID.objects.values('uuid_fk__id')))
+        self.assertCountEqual(qs, [u])
+
     def test_nested_subquery(self):
         inner = Company.objects.filter(point_of_contact=OuterRef('pk'))
         outer = Employee.objects.annotate(is_point_of_contact=Exists(inner))