diff --git a/tests/storage/test_relations.py b/tests/storage/test_relations.py
new file mode 100644
index 0000000000..cd1d00208b
--- /dev/null
+++ b/tests/storage/test_relations.py
@@ -0,0 +1,111 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from twisted.test.proto_helpers import MemoryReactor
+
+from synapse.api.constants import MAIN_TIMELINE
+from synapse.server import HomeServer
+from synapse.util import Clock
+
+from tests import unittest
+
+
+class RelationsStoreTestCase(unittest.HomeserverTestCase):
+ def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+ """
+ Creates a DAG:
+
+ A <---[m.thread]-- B <--[m.annotation]-- C
+ ^
+ |--[m.reference]-- D <--[m.annotation]-- E
+
+ F <--[m.annotation]-- G
+
+ """
+ self._main_store = self.hs.get_datastores().main
+
+ self._create_relation("A", "B", "m.thread")
+ self._create_relation("B", "C", "m.annotation")
+ self._create_relation("A", "D", "m.reference")
+ self._create_relation("D", "E", "m.annotation")
+ self._create_relation("F", "G", "m.annotation")
+
+ def _create_relation(self, parent_id: str, event_id: str, rel_type: str) -> None:
+ self.get_success(
+ self._main_store.db_pool.simple_insert(
+ table="event_relations",
+ values={
+ "event_id": event_id,
+ "relates_to_id": parent_id,
+ "relation_type": rel_type,
+ },
+ )
+ )
+
+ def test_get_thread_id(self) -> None:
+ """
+ Ensure that get_thread_id only searches up the tree for threads.
+ """
+ # The thread itself and children of it return the thread.
+ thread_id = self.get_success(self._main_store.get_thread_id("B"))
+ self.assertEqual("A", thread_id)
+
+ thread_id = self.get_success(self._main_store.get_thread_id("C"))
+ self.assertEqual("A", thread_id)
+
+ # But the root and events related to the root do not.
+ thread_id = self.get_success(self._main_store.get_thread_id("A"))
+ self.assertEqual(MAIN_TIMELINE, thread_id)
+
+ thread_id = self.get_success(self._main_store.get_thread_id("D"))
+ self.assertEqual(MAIN_TIMELINE, thread_id)
+
+ thread_id = self.get_success(self._main_store.get_thread_id("E"))
+ self.assertEqual(MAIN_TIMELINE, thread_id)
+
+ # Events which are not related to a thread at all should return the
+ # main timeline.
+ thread_id = self.get_success(self._main_store.get_thread_id("F"))
+ self.assertEqual(MAIN_TIMELINE, thread_id)
+
+ thread_id = self.get_success(self._main_store.get_thread_id("G"))
+ self.assertEqual(MAIN_TIMELINE, thread_id)
+
+ def test_get_thread_id_for_receipts(self) -> None:
+ """
+ Ensure that get_thread_id_for_receipts searches up and down the tree for a thread.
+ """
+ # All of the events are considered related to this thread.
+ thread_id = self.get_success(self._main_store.get_thread_id_for_receipts("A"))
+ self.assertEqual("A", thread_id)
+
+ thread_id = self.get_success(self._main_store.get_thread_id_for_receipts("B"))
+ self.assertEqual("A", thread_id)
+
+ thread_id = self.get_success(self._main_store.get_thread_id_for_receipts("C"))
+ self.assertEqual("A", thread_id)
+
+ thread_id = self.get_success(self._main_store.get_thread_id_for_receipts("D"))
+ self.assertEqual("A", thread_id)
+
+ thread_id = self.get_success(self._main_store.get_thread_id_for_receipts("E"))
+ self.assertEqual("A", thread_id)
+
+ # Events which are not related to a thread at all should return the
+ # main timeline.
+ thread_id = self.get_success(self._main_store.get_thread_id("F"))
+ self.assertEqual(MAIN_TIMELINE, thread_id)
+
+ thread_id = self.get_success(self._main_store.get_thread_id("G"))
+ self.assertEqual(MAIN_TIMELINE, thread_id)
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