Add a thread relation type per MSC3440. (#11088)

Adds experimental support for MSC3440's `io.element.thread` relation
type (and the aggregation for it).

1 files changed, 2 insertions, 1 deletions

diff --git a/synapse/rest/client/relations.py b/synapse/rest/client/relations.py

index d695c18be2..58f6699073 100644
--- a/synapse/rest/client/relations.py
+++ b/synapse/rest/client/relations.py
@@ -128,9 +128,10 @@ class RelationSendServlet(RestServlet):
 
         content["m.relates_to"] = {
             "event_id": parent_id,
-            "key": aggregation_key,
             "rel_type": relation_type,
         }
+        if aggregation_key is not None:
+            content["m.relates_to"]["key"] = aggregation_key
 
         event_dict = {
             "type": event_type,