Merge pull request #866 from bckohan/master

Fix database routing bug on save()

Author: bckohan

.github/workflows/test.yml

@@ -38,7 +38,6 @@ jobs:
         psycopg-version: ['psycopg2', 'psycopg3']
         django-version:
           - '4.2' # LTS April 2026
-          - '5.1' # December 2025
           - '5.2' # LTS April 2028
           - '6.0' # 
         exclude:
@@ -53,13 +52,9 @@ jobs:
 
           - python-version: '3.14'
             django-version: '4.2'
-          - python-version: '3.14'
-            django-version: '5.1'
           - python-version: '3.14'
             django-version: '5.2'
 
-          - postgres-version: '12'
-            django-version: '5.1'
           - postgres-version: '12'
             django-version: '5.2'
           - postgres-version: '12'
@@ -74,8 +69,6 @@ jobs:
 
           - postgres-version: 'latest'
             django-version: '4.2'
-          - postgres-version: 'latest'
-            django-version: '5.1'
 
           - postgres-version: '12'
             psycopg-version: 'psycopg3'

docs/changelog/index.rst

@@ -1,6 +1,11 @@
 Changelog
 =========
 
+v4.11.1 (2026-02-20)
+--------------------
+
+* Fixed `Release 4.11 causes UPDATE query to be routed to reader DB <https://github.com/jazzband/django-polymorphic/issues/865>`_
+
 v4.11.0 (2026-02-05)
 --------------------
 

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "django-polymorphic"
-version = "4.11.0"
+version = "4.11.1"
 description = "Seamless polymorphic inheritance for Django models."
 readme = "README.md"
 license = "BSD-3-Clause"
@@ -31,7 +31,6 @@ classifiers = [
     "Environment :: Web Environment",
     "Framework :: Django",
     "Framework :: Django :: 4.2",
-    "Framework :: Django :: 5.1",
     "Framework :: Django :: 5.2",
     "Framework :: Django :: 6.0",
     "Intended Audience :: Developers",

src/polymorphic/__init__.py

@@ -21,7 +21,7 @@
 
 from typing import Final
 
-VERSION: Final[str] = "4.11.0"
+VERSION: Final[str] = "4.11.1"
 
 __title__: Final = "Django Polymorphic"
 __version__ = VERSION  # version synonym for backwards compatibility

src/polymorphic/models.py

@@ -9,7 +9,7 @@
 from typing import ClassVar, cast
 
 from django.contrib.contenttypes.models import ContentType
-from django.db import models, transaction
+from django.db import models, router, transaction
 from django.db.models import Q
 from django.db.models.base import ModelBase
 from django.db.utils import DEFAULT_DB_ALIAS
@@ -127,15 +127,18 @@ def save(
         update_fields: Iterable[str] | None = None,
     ) -> None:
         """Calls :meth:`pre_save_polymorphic` and saves the model."""
-        # Determine the database to use:
+        # Determine the database to use via Django's routing infrastructure:
         # 1. Explicit 'using' parameter takes precedence
-        # 2. Otherwise use self._state.db (the database the object was loaded from)
-        # 3. Fall back to DEFAULT_DB_ALIAS
-        # This ensures database routers are respected when no explicit database is specified
-        if using is None:
-            using = self._state.db or DEFAULT_DB_ALIAS
-
-        self.pre_save_polymorphic(using=using)
+        # 2. Otherwise consult DATABASE_ROUTERS via router.db_for_write()
+        # 3. The router falls back to _state.db, then DEFAULT_DB_ALIAS
+        self.pre_save_polymorphic(
+            using=(
+                using
+                or router.db_for_write(self.__class__, instance=self)
+                or self._state.db
+                or DEFAULT_DB_ALIAS
+            )
+        )
         return super().save(
             force_insert=force_insert,
             force_update=force_update,

src/polymorphic/tests/test_multidb.py

@@ -332,3 +332,40 @@ def test_database_router_respected(self):
 
         # Verify it wasn't saved to default database
         self.assertFalse(Model2B.objects.filter(pk=obj.pk).exists())
+
+    def test_save_respects_db_for_write_router(self):
+        """
+        When a DATABASE_ROUTER routes writes to a different database than where
+        the object was read from, save() should respect db_for_write() rather
+        than blindly using _state.db. Regression test for issue #865.
+        """
+
+        class ReadWriteSplitRouter:
+            """Router that sends writes to 'default' regardless of _state.db."""
+
+            def db_for_read(self, model, **hints):
+                return None
+
+            def db_for_write(self, model, **hints):
+                return "default"
+
+            def allow_relation(self, obj1, obj2, **hints):
+                return True
+
+            def allow_migrate(self, db, app_label, **hints):
+                return True
+
+        # Create an object on "secondary", simulating an object loaded from a
+        # read-replica whose _state.db points at the replica.
+        obj = Model2B.objects.using("secondary").create(field1="test", field2="value")
+        self.assertEqual(obj._state.db, "secondary")
+
+        with self.settings(DATABASE_ROUTERS=[ReadWriteSplitRouter()]):
+            # save() without an explicit `using` should consult the router's
+            # db_for_write() and route to "default", not to _state.db ("secondary").
+            obj.field1 = "modified"
+            obj.save()
+
+        # The write should have landed in "default" (the write database).
+        self.assertTrue(Model2B.objects.using("default").filter(field1="modified").exists())
+        self.assertFalse(Model2B.objects.using("secondary").filter(field1="modified").exists())