Fixed #23338 -- Added warning when unique=True on ForeigKey

Thanks Jonathan Lindén for the initial patch, and Tim Graham
and Gabe Jackson for the suggestions.

2 files changed, 16 insertions, 0 deletions

diff --git a/django/contrib/auth/tests/test_management.py b/django/contrib/auth/tests/test_management.py
index 20f6c58f05..72d61a0919 100644
--- a/django/contrib/auth/tests/test_management.py
+++ b/django/contrib/auth/tests/test_management.py
@@ -153,6 +153,7 @@ class ChangepasswordManagementCommandTestCase(TestCase):
 
 
 @skipIfCustomUser
+@override_settings(SILENCED_SYSTEM_CHECKS=['fields.W342'])  # ForeignKey(unique=True)
 class CreatesuperuserManagementCommandTestCase(TestCase):
 
     def test_basic_usage(self):
diff --git a/django/db/models/fields/related.py b/django/db/models/fields/related.py
index ea88d19a17..04189ad097 100644
--- a/django/db/models/fields/related.py
+++ b/django/db/models/fields/related.py
@@ -1710,6 +1710,7 @@ class ForeignKey(ForeignObject):
     def check(self, **kwargs):
         errors = super(ForeignKey, self).check(**kwargs)
         errors.extend(self._check_on_delete())
+        errors.extend(self._check_unique())
         return errors
 
     def _check_on_delete(self):
@@ -1735,6 +1736,16 @@ class ForeignKey(ForeignObject):
         else:
             return []
 
+    def _check_unique(self, **kwargs):
+        return [
+            checks.Warning(
+                'Setting unique=True on a ForeignKey has the same effect as using a OneToOneField.',
+                hint='ForeignKey(unique=True) is usually better served by a OneToOneField.',
+                obj=self,
+                id='fields.W342',
+            )
+        ] if self.unique else []
+
     def deconstruct(self):
         name, path, args, kwargs = super(ForeignKey, self).deconstruct()
         del kwargs['to_fields']
@@ -1891,6 +1902,10 @@ class OneToOneField(ForeignKey):
         else:
             setattr(instance, self.attname, data)
 
+    def _check_unique(self, **kwargs):
+        # override ForeignKey since check isn't applicable here
+        return []
+
 
 def create_many_to_many_intermediary_model(field, klass):
     from django.db import models