diff --git a/nova/api/openstack/placement/objects/resource_provider.py b/nova/api/openstack/placement/objects/resource_provider.py
index e41bc128a296..85e5d8923a9f 100644
--- a/nova/api/openstack/placement/objects/resource_provider.py
+++ b/nova/api/openstack/placement/objects/resource_provider.py
@@ -1041,6 +1041,23 @@ class ResourceProvider(base.VersionedObject, base.TimestampedObject):
                             action='update',
                             reason=_('re-parenting a provider is not '
                                      'currently allowed.'))
+                if my_ids.parent_uuid is None:
+                    # So the user specifies a parent for an RP that doesn't
+                    # have one. We have to check that by this new parent we
+                    # don't create a loop in the tree. Basically the new parent
+                    # cannot be the RP itself or one of its descendants.
+                    # However as the RP's current parent is None the above
+                    # condition is the same as "the new parent cannot be any RP
+                    # from the current RP tree".
+                    same_tree = ResourceProviderList.get_all_by_filters(
+                        context,
+                        filters={'in_tree': self.uuid})
+                    rp_uuids_in_the_same_tree = [rp.uuid for rp in same_tree]
+                    if parent_uuid in rp_uuids_in_the_same_tree:
+                        raise exception.ObjectActionError(
+                            action='update',
+                            reason=_('creating loop in the provider tree is '
+                                     'not allowed.'))
 
                 updates['root_provider_id'] = parent_ids.root_id
                 updates['parent_provider_id'] = parent_ids.id
diff --git a/nova/tests/functional/api/openstack/placement/gabbits/resource-provider.yaml b/nova/tests/functional/api/openstack/placement/gabbits/resource-provider.yaml
index c025ddc80dee..98e32268cde4 100644
--- a/nova/tests/functional/api/openstack/placement/gabbits/resource-provider.yaml
+++ b/nova/tests/functional/api/openstack/placement/gabbits/resource-provider.yaml
@@ -731,6 +731,17 @@ tests:
   data:
       name: parent
       parent_provider_uuid: $ENVIRON['PARENT_PROVIDER_UUID']
-# this reproduces bug 1779635 as this should be HTTP 400 as we have a loop now
-# in the tree
-  status: 200
+  status: 400
+  response_strings:
+      - 'creating loop in the provider tree is not allowed.'
+
+- name: fail updating the parent to point to its child
+  PUT: /resource_providers/$ENVIRON['PARENT_PROVIDER_UUID']
+  request_headers:
+      content-type: application/json
+  data:
+      name: parent
+      parent_provider_uuid: $ENVIRON['RP_UUID']
+  status: 400
+  response_strings:
+      - 'creating loop in the provider tree is not allowed.'