From d90087cffa362cef13f6823df29d2c121a2f9bfb Mon Sep 17 00:00:00 2001
From: Patrick Cloke <clokep@users.noreply.github.com>
Date: Wed, 29 Jul 2020 13:55:01 -0400
Subject: [PATCH] Remove from the event_relations table when purging historical
 events. (#7978)

---
 changelog.d/7978.bugfix                          | 1 +
 synapse/storage/data_stores/main/purge_events.py | 2 ++
 2 files changed, 3 insertions(+)
 create mode 100644 changelog.d/7978.bugfix

diff --git a/changelog.d/7978.bugfix b/changelog.d/7978.bugfix
new file mode 100644
index 000000000..247b18db2
--- /dev/null
+++ b/changelog.d/7978.bugfix
@@ -0,0 +1 @@
+Fix a long standing bug: 'Duplicate key value violates unique constraint "event_relations_id"' when message retention is configured.
diff --git a/synapse/storage/data_stores/main/purge_events.py b/synapse/storage/data_stores/main/purge_events.py
index 654656913..b53fe35c3 100644
--- a/synapse/storage/data_stores/main/purge_events.py
+++ b/synapse/storage/data_stores/main/purge_events.py
@@ -62,6 +62,7 @@ class PurgeEventsStore(StateGroupWorkerStore, SQLBaseStore):
         #     event_json
         #     event_push_actions
         #     event_reference_hashes
+        #     event_relations
         #     event_search
         #     event_to_state_groups
         #     events
@@ -209,6 +210,7 @@ class PurgeEventsStore(StateGroupWorkerStore, SQLBaseStore):
             "event_edges",
             "event_forward_extremities",
             "event_reference_hashes",
+            "event_relations",
             "event_search",
             "rejections",
         ):