[1.10.x] Fixed #27027 -- Restored Client.force_login() defaulting to the first auth backend.

Backport of fc8f097117 from master

django/test/client.py

@@ -626,6 +626,9 @@ class Client(RequestFactory):
             return False
 
     def force_login(self, user, backend=None):
+        if backend is None:
+            backend = settings.AUTHENTICATION_BACKENDS[0]
+        user.backend = backend
         self._login(user, backend)
 
     def _login(self, user, backend=None):

docs/releases/1.10.1.txt

@@ -33,3 +33,7 @@ Bugfixes
 * Prevented the ``migrate`` command from raising
   ``InconsistentMigrationHistory`` in the presence of unapplied squashed
   migrations (:ticket:`27004`).
+
+* Fixed a regression in ``Client.force_login()`` which required specifying a
+  ``backend`` rather than automatically using the first one if multiple
+  backends are configured (:ticket:`27027`).

tests/test_client/tests.py

@@ -532,12 +532,30 @@ class ClientTest(TestCase):
 
         # Log in
         self.client.force_login(self.u1, backend='test_client.auth_backends.TestClientBackend')
+        self.assertEqual(self.u1.backend, 'test_client.auth_backends.TestClientBackend')
 
         # Request a page that requires a login
         response = self.client.get('/login_protected_view/')
         self.assertEqual(response.status_code, 200)
         self.assertEqual(response.context['user'].username, 'testclient')
 
+    @override_settings(
+        AUTHENTICATION_BACKENDS=[
+            'django.contrib.auth.backends.ModelBackend',
+            'test_client.auth_backends.TestClientBackend',
+        ],
+    )
+    def test_force_login_without_backend(self):
+        """
+        force_login() without passing a backend and with multiple backends
+        configured should automatically use the first backend.
+        """
+        self.client.force_login(self.u1)
+        response = self.client.get('/login_protected_view/')
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.context['user'].username, 'testclient')
+        self.assertEqual(self.u1.backend, 'django.contrib.auth.backends.ModelBackend')
+
     @override_settings(SESSION_ENGINE="django.contrib.sessions.backends.signed_cookies")
     def test_logout_cookie_sessions(self):
         self.test_logout()