[5.2.x] Refs #33647 -- Fixed silent data truncation in bulk_create on Postgres.

Regression in a16eedcf9c69d8a11d94cac1811018c5b996d491.

The UNNEST strategy is affected by the same problem bulk_update has wrt/
to silent data truncation due to its usage of db_type which always returns
a parametrized subtype.

Backport of d6ae2ed868e43671afc4d433c3d8f4d27f7eb555 from main.

3 files changed, 18 insertions, 2 deletions

diff --git a/django/db/backends/postgresql/compiler.py b/django/db/backends/postgresql/compiler.py
index 38b61c4898..fcd7faaf35 100644
--- a/django/db/backends/postgresql/compiler.py
+++ b/django/db/backends/postgresql/compiler.py
@@ -62,7 +62,9 @@ class SQLInsertCompiler(BaseSQLInsertCompiler):
             or any(any(hasattr(value, "as_sql") for value in row) for row in value_rows)
         ):
             return super().assemble_as_sql(fields, value_rows)
-        db_types = [field.db_type(self.connection) for field in fields]
+        # Manually remove parameters from `db_type` to ensure no data
+        # truncation takes place (e.g. varchar[] instead of varchar(50)[]).
+        db_types = [field.db_type(self.connection).split("(")[0] for field in fields]
         return InsertUnnest(["(%%s)::%s[]" % db_type for db_type in db_types]), [
             list(map(list, zip(*value_rows)))
         ]
diff --git a/docs/releases/5.2.10.txt b/docs/releases/5.2.10.txt
index 0b7ab06d54..6be8f79885 100644
--- a/docs/releases/5.2.10.txt
+++ b/docs/releases/5.2.10.txt
@@ -4,11 +4,15 @@ Django 5.2.10 release notes
 
 *Expected January 6, 2026*
 
-Django 5.2.10 fixes several bugs in 5.2.9.
+Django 5.2.10 fixes a data loss bug in Django 5.2 and one bug related to
+support for Python 3.14.
 
 Bugfixes
 ========
 
+* Fixed a bug in Django 5.2 where data exceeding ``max_length`` was silently
+  truncated by :meth:`.QuerySet.bulk_create` on PostgreSQL (:ticket:`33647`).
+
 * Fixed a bug where management command colorized help (introduced in
   Python 3.14) ignored the :option:`--no-color` option and the
   :envvar:`DJANGO_COLORS` setting (:ticket:`36376`).
diff --git a/tests/backends/postgresql/test_compilation.py b/tests/backends/postgresql/test_compilation.py
index 11c4015e01..2053c18a1a 100644
--- a/tests/backends/postgresql/test_compilation.py
+++ b/tests/backends/postgresql/test_compilation.py
@@ -3,6 +3,7 @@ from datetime import date
 
 from django.db import connection
 from django.db.models.expressions import RawSQL
+from django.db.utils import DataError
 from django.test import TestCase
 
 from ..models import Article, Reporter, Square
@@ -42,3 +43,12 @@ class BulkCreateUnnestTests(TestCase):
         self.assertEqual(
             [article.reporter for article in articles], [reporter, reporter]
         )
+
+    def test_parametrized_db_type(self):
+        with self.assertRaises(DataError):
+            Reporter.objects.bulk_create(
+                [
+                    Reporter(),
+                    Reporter(first_name="a" * 31),
+                ]
+            )