Allow resetting product version

Resetting base products was implemented in previous patch. Product
version should have been updated to work the same way, but was
forgotten. Test is added.

JIRA: PDC-1095

pdc/apps/release/serializers.py

@@ -63,7 +63,8 @@ class ReleaseSerializer(StrictSerializerMixin, serializers.ModelSerializer):
                                                 allow_null=True)
     product_version = serializers.SlugRelatedField(slug_field='product_version_id',
                                                    queryset=ProductVersion.objects.all(),
-                                                   required=False)
+                                                   required=False,
+                                                   allow_null=True)
     active = serializers.BooleanField(default=True)
     integrated_with = serializers.SlugRelatedField(slug_field='release_id',
                                                    queryset=Release.objects.all(),

pdc/apps/release/tests.py

@@ -1013,6 +1013,23 @@ def test_update_can_reset_base_product(self):
         release = models.Release.objects.get(release_id='release-1.0')
         self.assertIsNone(release.base_product)
 
+    def test_update_can_reset_product_version(self):
+        self.release.product_version = models.ProductVersion.objects.create(
+            name='Base Product',
+            short='p',
+            version='1',
+            product=models.Product.objects.create(name='Product', short='p')
+        )
+        self.release.save()
+
+        response = self.client.patch(reverse('release-detail', args=['release-1.0']),
+                                     {'product_version': None}, format='json')
+        self.assertEqual(response.status_code, status.HTTP_200_OK)
+        self.assertNumChanges([1])
+        self.assertIsNone(response.data['product_version'])
+        release = models.Release.objects.get(release_id='release-1.0')
+        self.assertIsNone(release.product_version)
+
     def test_update_can_explicitly_erase_optional_field_via_patch(self):
         response = self.client.patch(self.url, {'dist_git': None}, format='json')
         self.assertEqual(response.status_code, status.HTTP_200_OK)