Fixed #24910 -- Added createsuperuser support for non-unique USERNAME_FIELDs

Clarified docs to say that a non-unique USERNAME_FIELD is permissable
as long as the custom auth backend can support it.

tests/auth_tests/models/invalid_models.py

@@ -1,12 +1,23 @@
-from django.contrib.auth.models import AbstractBaseUser
+from django.contrib.auth.models import AbstractBaseUser, UserManager
 from django.db import models
 
 
 class CustomUserNonUniqueUsername(AbstractBaseUser):
-    "A user with a non-unique username"
+    """
+    A user with a non-unique username.
+
+    This model is not invalid if it is used with a custom authentication
+    backend which supports non-unique usernames.
+    """
     username = models.CharField(max_length=30)
+    email = models.EmailField(blank=True)
+    is_staff = models.BooleanField(default=False)
+    is_superuser = models.BooleanField(default=False)
 
     USERNAME_FIELD = 'username'
+    REQUIRED_FIELDS = ['email']
+
+    objects = UserManager()
 
     class Meta:
         app_label = 'auth'

tests/auth_tests/test_management.py

@@ -305,6 +305,33 @@ class CreatesuperuserManagementCommandTestCase(TestCase):
 
         self.assertEqual(CustomUser._default_manager.count(), 0)
 
+    @override_settings(
+        AUTH_USER_MODEL='auth.CustomUserNonUniqueUsername',
+        AUTHENTICATION_BACKENDS=['my.custom.backend'],
+    )
+    def test_swappable_user_username_non_unique(self):
+        @mock_inputs({
+            'username': 'joe',
+            'password': 'nopasswd',
+        })
+        def createsuperuser():
+            new_io = six.StringIO()
+            call_command(
+                "createsuperuser",
+                interactive=True,
+                email="joe@somewhere.org",
+                stdout=new_io,
+                stdin=MockTTY(),
+            )
+            command_output = new_io.getvalue().strip()
+            self.assertEqual(command_output, 'Superuser created successfully.')
+
+        for i in range(2):
+            createsuperuser()
+
+        users = CustomUserNonUniqueUsername.objects.filter(username="joe")
+        self.assertEqual(users.count(), 2)
+
     def test_skip_if_not_in_TTY(self):
         """
         If the command is not called from a TTY, it should be skipped and a