[5.0.x] Fixed #34840 -- Avoided casting string base fields on PostgreSQL.

Thanks Alex Vandiver for the report.

Regression in 09ffc5c1212d4ced58b708cbbf3dfbfb77b782ca.

Backport of 779cd28acb1f7eb06f629c0ea4ded99b5ebb670a from main.

1 files changed, 14 insertions, 0 deletions

diff --git a/tests/backends/postgresql/tests.py b/tests/backends/postgresql/tests.py
index 947d51ea1e..3a1d2da35d 100644
--- a/tests/backends/postgresql/tests.py
+++ b/tests/backends/postgresql/tests.py
@@ -376,6 +376,20 @@ class Tests(TestCase):
                         "::citext", do.lookup_cast(lookup, internal_type=field_type)
                     )
 
+    def test_lookup_cast_isnull_noop(self):
+        from django.db.backends.postgresql.operations import DatabaseOperations
+
+        do = DatabaseOperations(connection=None)
+        # Using __isnull lookup doesn't require casting.
+        tests = [
+            "CharField",
+            "EmailField",
+            "TextField",
+        ]
+        for field_type in tests:
+            with self.subTest(field_type=field_type):
+                self.assertEqual(do.lookup_cast("isnull", field_type), "%s")
+
     def test_correct_extraction_psycopg_version(self):
         from django.db.backends.postgresql.base import Database, psycopg_version