diff --git a/AUTHORS b/AUTHORS
index 59420932d55..3224be14a59 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -763,6 +763,7 @@ answer newbie questions, and generally made Django that much better:
     Stanislaus Madueke
     Stanislav Karpov <work@stkrp.ru>
     starrynight <cmorgh@gmail.com>
+    Stefan R. Filipek
     Stefane Fermgier <sf@fermigier.com>
     Stefano Rivera <stefano@rivera.za.net>
     Stéphane Raimbault <stephane.raimbault@gmail.com>
diff --git a/django/db/models/fields/related_descriptors.py b/django/db/models/fields/related_descriptors.py
index f9b770fadef..850ea99fa1a 100644
--- a/django/db/models/fields/related_descriptors.py
+++ b/django/db/models/fields/related_descriptors.py
@@ -205,11 +205,10 @@ class ForwardManyToOneDescriptor:
         elif value is not None:
             if instance._state.db is None:
                 instance._state.db = router.db_for_write(instance.__class__, instance=value)
-            elif value._state.db is None:
+            if value._state.db is None:
                 value._state.db = router.db_for_write(value.__class__, instance=instance)
-            elif value._state.db is not None and instance._state.db is not None:
-                if not router.allow_relation(value, instance):
-                    raise ValueError('Cannot assign "%r": the current database router prevents this relation.' % value)
+            if not router.allow_relation(value, instance):
+                raise ValueError('Cannot assign "%r": the current database router prevents this relation.' % value)
 
         remote_field = self.field.remote_field
         # If we're setting the value of a OneToOneField to None, we need to clear
@@ -451,11 +450,10 @@ class ReverseOneToOneDescriptor:
         else:
             if instance._state.db is None:
                 instance._state.db = router.db_for_write(instance.__class__, instance=value)
-            elif value._state.db is None:
+            if value._state.db is None:
                 value._state.db = router.db_for_write(value.__class__, instance=instance)
-            elif value._state.db is not None and instance._state.db is not None:
-                if not router.allow_relation(value, instance):
-                    raise ValueError('Cannot assign "%r": the current database router prevents this relation.' % value)
+            if not router.allow_relation(value, instance):
+                raise ValueError('Cannot assign "%r": the current database router prevents this relation.' % value)
 
             related_pk = tuple(getattr(instance, field.attname) for field in self.related.field.foreign_related_fields)
             # Set the value of the related field to the value of the related object's related field
diff --git a/docs/releases/2.1.txt b/docs/releases/2.1.txt
index 101d42ee037..5f23d6d241d 100644
--- a/docs/releases/2.1.txt
+++ b/docs/releases/2.1.txt
@@ -415,6 +415,9 @@ Miscellaneous
 * ``QuerySet.raw()`` now caches its results like regular querysets. Use
   ``iterator()`` if you don't want caching.
 
+* The database router :meth:`allow_relation` method is called in more cases.
+  Improperly written routers may need to be updated accordingly.
+
 .. _deprecated-features-2.1:
 
 Features deprecated in 2.1
diff --git a/tests/gis_tests/layermap/tests.py b/tests/gis_tests/layermap/tests.py
index 1ebeeb31c6c..2406533e666 100644
--- a/tests/gis_tests/layermap/tests.py
+++ b/tests/gis_tests/layermap/tests.py
@@ -319,7 +319,10 @@ class OtherRouter:
         return self.db_for_read(model, **hints)
 
     def allow_relation(self, obj1, obj2, **hints):
-        return None
+        # ContentType objects are created during a post-migrate signal while
+        # performing fixture teardown using the default database alias and
+        # don't abide by the database specified by this router.
+        return True
 
     def allow_migrate(self, db, app_label, **hints):
         return True
diff --git a/tests/multiple_database/tests.py b/tests/multiple_database/tests.py
index 6dd44f64750..e4cef8cdb94 100644
--- a/tests/multiple_database/tests.py
+++ b/tests/multiple_database/tests.py
@@ -2082,3 +2082,28 @@ class RouteForWriteTestCase(TestCase):
         self.assertEqual(e.mode, RouterUsed.WRITE)
         self.assertEqual(e.model, Book)
         self.assertEqual(e.hints, {'instance': auth})
+
+
+class NoRelationRouter:
+    """Disallow all relations."""
+    def allow_relation(self, obj1, obj2, **hints):
+        return False
+
+
+@override_settings(DATABASE_ROUTERS=[NoRelationRouter()])
+class RelationAssignmentTests(TestCase):
+    """allow_relation() is called with unsaved model instances."""
+    multi_db = True
+    router_prevents_msg = 'the current database router prevents this relation'
+
+    def test_foreign_key_relation(self):
+        person = Person(name='Someone')
+        pet = Pet()
+        with self.assertRaisesMessage(ValueError, self.router_prevents_msg):
+            pet.owner = person
+
+    def test_reverse_one_to_one_relation(self):
+        user = User(username='Someone', password='fake_hash')
+        profile = UserProfile()
+        with self.assertRaisesMessage(ValueError, self.router_prevents_msg):
+            user.userprofile = profile