Fixed #31277 -- Relaxed system check of m2m intermediary tables for db_table collision when database routers are installed.

Turned the error into a warning when database routers are installed.

AUTHORS

@@ -931,6 +931,7 @@ answer newbie questions, and generally made Django that much better:
     Wilson Miner <wminer@gmail.com>
     Wim Glenn <hey@wimglenn.com>
     wojtek
+    Xavier Francisco <xavier.n.francisco@gmail.com>
     Xia Kai <https://blog.xiaket.org/>
     Yann Fouillat <gagaro42@gmail.com>
     Yann Malet

django/db/models/fields/related.py

@@ -4,7 +4,7 @@ from functools import partial
 
 from django import forms
 from django.apps import apps
-from django.conf import SettingsReference
+from django.conf import SettingsReference, settings
 from django.core import checks, exceptions
 from django.db import connection, router
 from django.db.backends import utils
@@ -1436,12 +1436,23 @@ class ManyToManyField(RelatedField):
                 clashing_obj = '%s.%s' % (opts.label, _get_field_name(model))
             else:
                 clashing_obj = model._meta.label
+            if settings.DATABASE_ROUTERS:
+                error_class, error_id = checks.Warning, 'fields.W344'
+                error_hint = (
+                    'You have configured settings.DATABASE_ROUTERS. Verify '
+                    'that the table of %r is correctly routed to a separate '
+                    'database.' % clashing_obj
+                )
+            else:
+                error_class, error_id = checks.Error, 'fields.E340'
+                error_hint = None
             return [
-                checks.Error(
+                error_class(
                     "The field's intermediary table '%s' clashes with the "
                     "table name of '%s'." % (m2m_db_table, clashing_obj),
                     obj=self,
-                    id='fields.E340',
+                    hint=error_hint,
+                    id=error_id,
                 )
             ]
         return []

docs/ref/checks.txt

@@ -261,6 +261,8 @@ Related fields
   effect as using a ``OneToOneField``.
 * **fields.W343**: ``limit_choices_to`` has no effect on ``ManyToManyField``
   with a ``through`` model.
+* **fields.W344**: The field's intermediary table ``<table name>`` clashes with
+  the table name of ``<model>``/``<model>.<field name>``.
 
 Models
 ------

tests/invalid_models_tests/test_models.py

@@ -10,6 +10,10 @@ from django.test import SimpleTestCase, TestCase
 from django.test.utils import isolate_apps, override_settings, register_lookup
 
 
+class EmptyRouter:
+    pass
+
+
 def get_max_column_name_length():
     allowed_len = None
     db_alias = None
@@ -1044,6 +1048,32 @@ class OtherModelTests(SimpleTestCase):
             )
         ])
 
+    @override_settings(DATABASE_ROUTERS=['invalid_models_tests.test_models.EmptyRouter'])
+    def test_m2m_table_name_clash_database_routers_installed(self):
+        class Foo(models.Model):
+            bar = models.ManyToManyField('Bar', db_table='myapp_bar')
+
+            class Meta:
+                db_table = 'myapp_foo'
+
+        class Bar(models.Model):
+            class Meta:
+                db_table = 'myapp_bar'
+
+        self.assertEqual(Foo.check(), [
+            Warning(
+                "The field's intermediary table 'myapp_bar' clashes with the "
+                "table name of 'invalid_models_tests.Bar'.",
+                obj=Foo._meta.get_field('bar'),
+                hint=(
+                    "You have configured settings.DATABASE_ROUTERS. Verify "
+                    "that the table of 'invalid_models_tests.Bar' is "
+                    "correctly routed to a separate database."
+                ),
+                id='fields.W344',
+            ),
+        ])
+
     def test_m2m_field_table_name_clash(self):
         class Foo(models.Model):
             pass
@@ -1069,6 +1099,32 @@ class OtherModelTests(SimpleTestCase):
             )
         ])
 
+    @override_settings(DATABASE_ROUTERS=['invalid_models_tests.test_models.EmptyRouter'])
+    def test_m2m_field_table_name_clash_database_routers_installed(self):
+        class Foo(models.Model):
+            pass
+
+        class Bar(models.Model):
+            foos = models.ManyToManyField(Foo, db_table='clash')
+
+        class Baz(models.Model):
+            foos = models.ManyToManyField(Foo, db_table='clash')
+
+        self.assertEqual(Bar.check() + Baz.check(), [
+            Warning(
+                "The field's intermediary table 'clash' clashes with the "
+                "table name of 'invalid_models_tests.%s.foos'."
+                % clashing_model,
+                obj=model_cls._meta.get_field('foos'),
+                hint=(
+                    "You have configured settings.DATABASE_ROUTERS. Verify "
+                    "that the table of 'invalid_models_tests.%s.foos' is "
+                    "correctly routed to a separate database." % clashing_model
+                ),
+                id='fields.W344',
+            ) for model_cls, clashing_model in [(Bar, 'Baz'), (Baz, 'Bar')]
+        ])
+
     def test_m2m_autogenerated_table_name_clash(self):
         class Foo(models.Model):
             class Meta:
@@ -1090,6 +1146,33 @@ class OtherModelTests(SimpleTestCase):
             )
         ])
 
+    @override_settings(DATABASE_ROUTERS=['invalid_models_tests.test_models.EmptyRouter'])
+    def test_m2m_autogenerated_table_name_clash_database_routers_installed(self):
+        class Foo(models.Model):
+            class Meta:
+                db_table = 'bar_foos'
+
+        class Bar(models.Model):
+            # The autogenerated db_table is bar_foos.
+            foos = models.ManyToManyField(Foo)
+
+            class Meta:
+                db_table = 'bar'
+
+        self.assertEqual(Bar.check(), [
+            Warning(
+                "The field's intermediary table 'bar_foos' clashes with the "
+                "table name of 'invalid_models_tests.Foo'.",
+                obj=Bar._meta.get_field('foos'),
+                hint=(
+                    "You have configured settings.DATABASE_ROUTERS. Verify "
+                    "that the table of 'invalid_models_tests.Foo' is "
+                    "correctly routed to a separate database."
+                ),
+                id='fields.W344',
+            ),
+        ])
+
     def test_m2m_unmanaged_shadow_models_not_checked(self):
         class A1(models.Model):
             pass