[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.

3 files changed, 60 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
 
diff --git a/tests/constraints/tests.py b/tests/constraints/tests.py
index 55397449d9..e1b95baf60 100644
--- a/tests/constraints/tests.py
+++ b/tests/constraints/tests.py
@@ -995,6 +995,42 @@ class UniqueConstraintTests(TestCase):
             exclude={"name"},
         )
 
+    def test_validate_nullable_textfield_with_isnull_true(self):
+        is_null_constraint = models.UniqueConstraint(
+            "price",
+            "discounted_price",
+            condition=models.Q(unit__isnull=True),
+            name="uniq_prices_no_unit",
+        )
+        is_not_null_constraint = models.UniqueConstraint(
+            "price",
+            "discounted_price",
+            condition=models.Q(unit__isnull=False),
+            name="uniq_prices_unit",
+        )
+
+        Product.objects.create(price=2, discounted_price=1)
+        Product.objects.create(price=4, discounted_price=3, unit="ng/mL")
+
+        msg = "Constraint “uniq_prices_no_unit” is violated."
+        with self.assertRaisesMessage(ValidationError, msg):
+            is_null_constraint.validate(
+                Product, Product(price=2, discounted_price=1, unit=None)
+            )
+        is_null_constraint.validate(
+            Product, Product(price=2, discounted_price=1, unit="ng/mL")
+        )
+        is_null_constraint.validate(Product, Product(price=4, discounted_price=3))
+
+        msg = "Constraint “uniq_prices_unit” is violated."
+        with self.assertRaisesMessage(ValidationError, msg):
+            is_not_null_constraint.validate(
+                Product,
+                Product(price=4, discounted_price=3, unit="μg/mL"),
+            )
+        is_not_null_constraint.validate(Product, Product(price=4, discounted_price=3))
+        is_not_null_constraint.validate(Product, Product(price=2, discounted_price=1))
+
     def test_name(self):
         constraints = get_constraints(UniqueConstraintProduct._meta.db_table)
         expected_name = "name_color_uniq"
diff --git a/tests/lookup/tests.py b/tests/lookup/tests.py
index 398cc89cab..8af459ccd7 100644
--- a/tests/lookup/tests.py
+++ b/tests/lookup/tests.py
@@ -1337,6 +1337,16 @@ class LookupTests(TestCase):
                 with self.assertRaisesMessage(ValueError, msg):
                     qs.exists()
 
+    def test_isnull_textfield(self):
+        self.assertSequenceEqual(
+            Author.objects.filter(bio__isnull=True),
+            [self.au2],
+        )
+        self.assertSequenceEqual(
+            Author.objects.filter(bio__isnull=False),
+            [self.au1],
+        )
+
     def test_lookup_rhs(self):
         product = Product.objects.create(name="GME", qty_target=5000)
         stock_1 = Stock.objects.create(product=product, short=True, qty_available=180)