Don't raise an exception when accessing a Relationship where null=True

Author: AlexHill

relativity/fields.py

@@ -281,7 +281,14 @@ def related_manager_cls(self):
 
 
 class SingleRelationshipDescriptor(ReverseOneToOneDescriptor):
-    pass
+    def __get__(self, instance, cls=None):
+        try:
+            return super(SingleRelationshipDescriptor, self).__get__(instance, cls=None)
+        except self.RelatedObjectDoesNotExist:
+            if self.related.null:
+                return None
+            else:
+                raise
 
 
 # noinspection PyProtectedMember

tests/models.py

@@ -139,7 +139,11 @@ class CartItem(models.Model):
     description = models.TextField()
 
     product = Relationship(
-        Product, Q(sku=L("product_code")), related_name="cart_items", multiple=False
+        Product,
+        Q(sku=L("product_code")),
+        related_name="cart_items",
+        multiple=False,
+        null=False,
     )
 
     def __str__(self):

tests/tests.py

@@ -356,6 +356,18 @@ def test_single_reverse(self):
         self.assertEqual(node_3.prev, node_2)
         self.assertEqual(node_1.next, node_2)
 
+        self.assertIsNone(node_3.next)
+        self.assertIsNone(node_1.prev)
+
+    def test_not_nullable(self):
+        item = CartItem.objects.create(
+            pk=4,
+            product_code="nonexistent",
+            description="cart item for anonexistent product",
+        )
+        with self.assertRaises(Product.DoesNotExist):
+            item.product
+
     def test_complex_expression(self):
         ug = UserGenerator.objects.create()
         self.assertEqual(ug.user, User.objects.get(username="generated_for_%d" % ug.id))