Fixed #26170 -- Made ModelAdmin views run transactions on the correct database.

Thanks juntatalor for the initial patch.

tests/auth_tests/test_admin_multidb.py

@@ -0,0 +1,49 @@
+from django.conf.urls import url
+from django.contrib import admin
+from django.contrib.auth.admin import UserAdmin
+from django.contrib.auth.models import User
+from django.db import connections
+from django.test import TestCase, mock, override_settings
+from django.urls import reverse
+
+
+class Router(object):
+    target_db = None
+
+    def db_for_read(self, model, **hints):
+        return self.target_db
+
+    db_for_write = db_for_read
+
+site = admin.AdminSite(name='test_adminsite')
+site.register(User, admin_class=UserAdmin)
+
+urlpatterns = [
+    url(r'^admin/', site.urls),
+]
+
+
+@override_settings(ROOT_URLCONF=__name__, DATABASE_ROUTERS=['%s.Router' % __name__])
+class MultiDatabaseTests(TestCase):
+    multi_db = True
+
+    @classmethod
+    def setUpTestData(cls):
+        cls.superusers = {}
+        for db in connections:
+            Router.target_db = db
+            cls.superusers[db] = User.objects.create_superuser(
+                username='admin', password='something', email='test@test.org',
+            )
+
+    @mock.patch('django.contrib.auth.admin.transaction')
+    def test_add_view(self, mock):
+        for db in connections:
+            Router.target_db = db
+            self.client.force_login(self.superusers[db])
+            self.client.post(reverse('test_adminsite:auth_user_add'), {
+                'username': 'some_user',
+                'password1': 'helloworld',
+                'password2': 'helloworld',
+            })
+            mock.atomic.assert_called_with(using=db)