Support filtering by relations per MSC3440 (#11236)

Adds experimental support for `relation_types` and `relation_senders`
fields for filters.

15 files changed, 680 insertions, 110 deletions

diff --git a/changelog.d/11236.feature b/changelog.d/11236.feature
new file mode 100644
index 0000000000..e7aeee2aa6
--- /dev/null
+++ b/changelog.d/11236.feature
@@ -0,0 +1 @@
+Support filtering by relation senders & types per [MSC3440](https://github.com/matrix-org/matrix-doc/pull/3440).
diff --git a/synapse/api/filtering.py b/synapse/api/filtering.py
index 4b0a9b2974..13dd6ce248 100644
--- a/synapse/api/filtering.py
+++ b/synapse/api/filtering.py
@@ -1,7 +1,7 @@
 # Copyright 2015, 2016 OpenMarket Ltd
 # Copyright 2017 Vector Creations Ltd
 # Copyright 2018-2019 New Vector Ltd
-# Copyright 2019 The Matrix.org Foundation C.I.C.
+# Copyright 2019-2021 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.
@@ -86,6 +86,9 @@ ROOM_EVENT_FILTER_SCHEMA = {
         # cf https://github.com/matrix-org/matrix-doc/pull/2326
         "org.matrix.labels": {"type": "array", "items": {"type": "string"}},
         "org.matrix.not_labels": {"type": "array", "items": {"type": "string"}},
+        # MSC3440, filtering by event relations.
+        "io.element.relation_senders": {"type": "array", "items": {"type": "string"}},
+        "io.element.relation_types": {"type": "array", "items": {"type": "string"}},
     },
 }
 
@@ -146,14 +149,16 @@ def matrix_user_id_validator(user_id_str: str) -> UserID:
 
 class Filtering:
     def __init__(self, hs: "HomeServer"):
-        super().__init__()
+        self._hs = hs
         self.store = hs.get_datastore()
 
+        self.DEFAULT_FILTER_COLLECTION = FilterCollection(hs, {})
+
     async def get_user_filter(
         self, user_localpart: str, filter_id: Union[int, str]
     ) -> "FilterCollection":
         result = await self.store.get_user_filter(user_localpart, filter_id)
-        return FilterCollection(result)
+        return FilterCollection(self._hs, result)
 
     def add_user_filter(
         self, user_localpart: str, user_filter: JsonDict
@@ -191,21 +196,22 @@ FilterEvent = TypeVar("FilterEvent", EventBase, UserPresenceState, JsonDict)
 
 
 class FilterCollection:
-    def __init__(self, filter_json: JsonDict):
+    def __init__(self, hs: "HomeServer", filter_json: JsonDict):
         self._filter_json = filter_json
 
         room_filter_json = self._filter_json.get("room", {})
 
         self._room_filter = Filter(
-            {k: v for k, v in room_filter_json.items() if k in ("rooms", "not_rooms")}
+            hs,
+            {k: v for k, v in room_filter_json.items() if k in ("rooms", "not_rooms")},
         )
 
-        self._room_timeline_filter = Filter(room_filter_json.get("timeline", {}))
-        self._room_state_filter = Filter(room_filter_json.get("state", {}))
-        self._room_ephemeral_filter = Filter(room_filter_json.get("ephemeral", {}))
-        self._room_account_data = Filter(room_filter_json.get("account_data", {}))
-        self._presence_filter = Filter(filter_json.get("presence", {}))
-        self._account_data = Filter(filter_json.get("account_data", {}))
+        self._room_timeline_filter = Filter(hs, room_filter_json.get("timeline", {}))
+        self._room_state_filter = Filter(hs, room_filter_json.get("state", {}))
+        self._room_ephemeral_filter = Filter(hs, room_filter_json.get("ephemeral", {}))
+        self._room_account_data = Filter(hs, room_filter_json.get("account_data", {}))
+        self._presence_filter = Filter(hs, filter_json.get("presence", {}))
+        self._account_data = Filter(hs, filter_json.get("account_data", {}))
 
         self.include_leave = filter_json.get("room", {}).get("include_leave", False)
         self.event_fields = filter_json.get("event_fields", [])
@@ -232,25 +238,37 @@ class FilterCollection:
     def include_redundant_members(self) -> bool:
         return self._room_state_filter.include_redundant_members
 
-    def filter_presence(
+    async def filter_presence(
         self, events: Iterable[UserPresenceState]
     ) -> List[UserPresenceState]:
-        return self._presence_filter.filter(events)
+        return await self._presence_filter.filter(events)
 
-    def filter_account_data(self, events: Iterable[JsonDict]) -> List[JsonDict]:
-        return self._account_data.filter(events)
+    async def filter_account_data(self, events: Iterable[JsonDict]) -> List[JsonDict]:
+        return await self._account_data.filter(events)
 
-    def filter_room_state(self, events: Iterable[EventBase]) -> List[EventBase]:
-        return self._room_state_filter.filter(self._room_filter.filter(events))
+    async def filter_room_state(self, events: Iterable[EventBase]) -> List[EventBase]:
+        return await self._room_state_filter.filter(
+            await self._room_filter.filter(events)
+        )
 
-    def filter_room_timeline(self, events: Iterable[EventBase]) -> List[EventBase]:
-        return self._room_timeline_filter.filter(self._room_filter.filter(events))
+    async def filter_room_timeline(
+        self, events: Iterable[EventBase]
+    ) -> List[EventBase]:
+        return await self._room_timeline_filter.filter(
+            await self._room_filter.filter(events)
+        )
 
-    def filter_room_ephemeral(self, events: Iterable[JsonDict]) -> List[JsonDict]:
-        return self._room_ephemeral_filter.filter(self._room_filter.filter(events))
+    async def filter_room_ephemeral(self, events: Iterable[JsonDict]) -> List[JsonDict]:
+        return await self._room_ephemeral_filter.filter(
+            await self._room_filter.filter(events)
+        )
 
-    def filter_room_account_data(self, events: Iterable[JsonDict]) -> List[JsonDict]:
-        return self._room_account_data.filter(self._room_filter.filter(events))
+    async def filter_room_account_data(
+        self, events: Iterable[JsonDict]
+    ) -> List[JsonDict]:
+        return await self._room_account_data.filter(
+            await self._room_filter.filter(events)
+        )
 
     def blocks_all_presence(self) -> bool:
         return (
@@ -274,7 +292,9 @@ class FilterCollection:
 
 
 class Filter:
-    def __init__(self, filter_json: JsonDict):
+    def __init__(self, hs: "HomeServer", filter_json: JsonDict):
+        self._hs = hs
+        self._store = hs.get_datastore()
         self.filter_json = filter_json
 
         self.limit = filter_json.get("limit", 10)
@@ -297,6 +317,20 @@ class Filter:
         self.labels = filter_json.get("org.matrix.labels", None)
         self.not_labels = filter_json.get("org.matrix.not_labels", [])
 
+        # Ideally these would be rejected at the endpoint if they were provided
+        # and not supported, but that would involve modifying the JSON schema
+        # based on the homeserver configuration.
+        if hs.config.experimental.msc3440_enabled:
+            self.relation_senders = self.filter_json.get(
+                "io.element.relation_senders", None
+            )
+            self.relation_types = self.filter_json.get(
+                "io.element.relation_types", None
+            )
+        else:
+            self.relation_senders = None
+            self.relation_types = None
+
     def filters_all_types(self) -> bool:
         return "*" in self.not_types
 
@@ -306,7 +340,7 @@ class Filter:
     def filters_all_rooms(self) -> bool:
         return "*" in self.not_rooms
 
-    def check(self, event: FilterEvent) -> bool:
+    def _check(self, event: FilterEvent) -> bool:
         """Checks whether the filter matches the given event.
 
         Args:
@@ -420,8 +454,30 @@ class Filter:
 
         return room_ids
 
-    def filter(self, events: Iterable[FilterEvent]) -> List[FilterEvent]:
-        return list(filter(self.check, events))
+    async def _check_event_relations(
+        self, events: Iterable[FilterEvent]
+    ) -> List[FilterEvent]:
+        # The event IDs to check, mypy doesn't understand the ifinstance check.
+        event_ids = [event.event_id for event in events if isinstance(event, EventBase)]  # type: ignore[attr-defined]
+        event_ids_to_keep = set(
+            await self._store.events_have_relations(
+                event_ids, self.relation_senders, self.relation_types
+            )
+        )
+
+        return [
+            event
+            for event in events
+            if not isinstance(event, EventBase) or event.event_id in event_ids_to_keep
+        ]
+
+    async def filter(self, events: Iterable[FilterEvent]) -> List[FilterEvent]:
+        result = [event for event in events if self._check(event)]
+
+        if self.relation_senders or self.relation_types:
+            return await self._check_event_relations(result)
+
+        return result
 
     def with_room_ids(self, room_ids: Iterable[str]) -> "Filter":
         """Returns a new filter with the given room IDs appended.
@@ -433,7 +489,7 @@ class Filter:
             filter: A new filter including the given rooms and the old
                     filter's rooms.
         """
-        newFilter = Filter(self.filter_json)
+        newFilter = Filter(self._hs, self.filter_json)
         newFilter.rooms += room_ids
         return newFilter
 
@@ -444,6 +500,3 @@ def _matches_wildcard(actual_value: Optional[str], filter_value: str) -> bool:
         return actual_value.startswith(type_prefix)
     else:
         return actual_value == filter_value
-
-
-DEFAULT_FILTER_COLLECTION = FilterCollection({})
diff --git a/synapse/handlers/pagination.py b/synapse/handlers/pagination.py
index abfe7be0e3..aa26911aed 100644
--- a/synapse/handlers/pagination.py
+++ b/synapse/handlers/pagination.py
@@ -424,7 +424,7 @@ class PaginationHandler:
 
         if events:
             if event_filter:
-                events = event_filter.filter(events)
+                events = await event_filter.filter(events)
 
             events = await filter_events_for_client(
                 self.storage, user_id, events, is_peeking=(member_event_id is None)
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 969eb3b9b0..7a95e31cbe 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -1158,8 +1158,10 @@ class RoomContextHandler:
         )
 
         if event_filter:
-            results["events_before"] = event_filter.filter(results["events_before"])
-            results["events_after"] = event_filter.filter(results["events_after"])
+            results["events_before"] = await event_filter.filter(
+                results["events_before"]
+            )
+            results["events_after"] = await event_filter.filter(results["events_after"])
 
         results["events_before"] = await filter_evts(results["events_before"])
         results["events_after"] = await filter_evts(results["events_after"])
@@ -1195,7 +1197,7 @@ class RoomContextHandler:
 
         state_events = list(state[last_event_id].values())
         if event_filter:
-            state_events = event_filter.filter(state_events)
+            state_events = await event_filter.filter(state_events)
 
         results["state"] = await filter_evts(state_events)
 
diff --git a/synapse/handlers/search.py b/synapse/handlers/search.py
index 6e4dff8056..ab7eaab2fb 100644
--- a/synapse/handlers/search.py
+++ b/synapse/handlers/search.py
@@ -180,7 +180,7 @@ class SearchHandler:
                 % (set(group_keys) - {"room_id", "sender"},),
             )
 
-        search_filter = Filter(filter_dict)
+        search_filter = Filter(self.hs, filter_dict)
 
         # TODO: Search through left rooms too
         rooms = await self.store.get_rooms_for_local_user_where_membership_is(
@@ -242,7 +242,7 @@ class SearchHandler:
 
             rank_map.update({r["event"].event_id: r["rank"] for r in results})
 
-            filtered_events = search_filter.filter([r["event"] for r in results])
+            filtered_events = await search_filter.filter([r["event"] for r in results])
 
             events = await filter_events_for_client(
                 self.storage, user.to_string(), filtered_events
@@ -292,7 +292,9 @@ class SearchHandler:
 
                 rank_map.update({r["event"].event_id: r["rank"] for r in results})
 
-                filtered_events = search_filter.filter([r["event"] for r in results])
+                filtered_events = await search_filter.filter(
+                    [r["event"] for r in results]
+                )
 
                 events = await filter_events_for_client(
                     self.storage, user.to_string(), filtered_events
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 2c7c6d63a9..891435c14d 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -510,7 +510,7 @@ class SyncHandler:
             log_kv({"limited": limited})
 
             if potential_recents:
-                recents = sync_config.filter_collection.filter_room_timeline(
+                recents = await sync_config.filter_collection.filter_room_timeline(
                     potential_recents
                 )
                 log_kv({"recents_after_sync_filtering": len(recents)})
@@ -575,8 +575,8 @@ class SyncHandler:
 
                 log_kv({"loaded_recents": len(events)})
 
-                loaded_recents = sync_config.filter_collection.filter_room_timeline(
-                    events
+                loaded_recents = (
+                    await sync_config.filter_collection.filter_room_timeline(events)
                 )
 
                 log_kv({"loaded_recents_after_sync_filtering": len(loaded_recents)})
@@ -1015,7 +1015,7 @@ class SyncHandler:
 
         return {
             (e.type, e.state_key): e
-            for e in sync_config.filter_collection.filter_room_state(
+            for e in await sync_config.filter_collection.filter_room_state(
                 list(state.values())
             )
             if e.type != EventTypes.Aliases  # until MSC2261 or alternative solution
@@ -1383,7 +1383,7 @@ class SyncHandler:
                 sync_config.user
             )
 
-        account_data_for_user = sync_config.filter_collection.filter_account_data(
+        account_data_for_user = await sync_config.filter_collection.filter_account_data(
             [
                 {"type": account_data_type, "content": content}
                 for account_data_type, content in account_data.items()
@@ -1448,7 +1448,7 @@ class SyncHandler:
             # Deduplicate the presence entries so that there's at most one per user
             presence = list({p.user_id: p for p in presence}.values())
 
-        presence = sync_config.filter_collection.filter_presence(presence)
+        presence = await sync_config.filter_collection.filter_presence(presence)
 
         sync_result_builder.presence = presence
 
@@ -2021,12 +2021,14 @@ class SyncHandler:
                 )
 
             account_data_events = (
-                sync_config.filter_collection.filter_room_account_data(
+                await sync_config.filter_collection.filter_room_account_data(
                     account_data_events
                 )
             )
 
-            ephemeral = sync_config.filter_collection.filter_room_ephemeral(ephemeral)
+            ephemeral = await sync_config.filter_collection.filter_room_ephemeral(
+                ephemeral
+            )
 
             if not (
                 always_include
diff --git a/synapse/rest/admin/rooms.py b/synapse/rest/admin/rooms.py
index 05c5b4bf0c..05c823d9ce 100644
--- a/synapse/rest/admin/rooms.py
+++ b/synapse/rest/admin/rooms.py
@@ -583,6 +583,7 @@ class RoomEventContextServlet(RestServlet):
 
     def __init__(self, hs: "HomeServer"):
         super().__init__()
+        self._hs = hs
         self.clock = hs.get_clock()
         self.room_context_handler = hs.get_room_context_handler()
         self._event_serializer = hs.get_event_client_serializer()
@@ -600,7 +601,9 @@ class RoomEventContextServlet(RestServlet):
         filter_str = parse_string(request, "filter", encoding="utf-8")
         if filter_str:
             filter_json = urlparse.unquote(filter_str)
-            event_filter: Optional[Filter] = Filter(json_decoder.decode(filter_json))
+            event_filter: Optional[Filter] = Filter(
+                self._hs, json_decoder.decode(filter_json)
+            )
         else:
             event_filter = None
 
diff --git a/synapse/rest/client/room.py b/synapse/rest/client/room.py
index 6a876cfa2f..03a353d53c 100644
--- a/synapse/rest/client/room.py
+++ b/synapse/rest/client/room.py
@@ -550,6 +550,7 @@ class RoomMessageListRestServlet(RestServlet):
 
     def __init__(self, hs: "HomeServer"):
         super().__init__()
+        self._hs = hs
         self.pagination_handler = hs.get_pagination_handler()
         self.auth = hs.get_auth()
         self.store = hs.get_datastore()
@@ -567,7 +568,9 @@ class RoomMessageListRestServlet(RestServlet):
         filter_str = parse_string(request, "filter", encoding="utf-8")
         if filter_str:
             filter_json = urlparse.unquote(filter_str)
-            event_filter: Optional[Filter] = Filter(json_decoder.decode(filter_json))
+            event_filter: Optional[Filter] = Filter(
+                self._hs, json_decoder.decode(filter_json)
+            )
             if (
                 event_filter
                 and event_filter.filter_json.get("event_format", "client")
@@ -672,6 +675,7 @@ class RoomEventContextServlet(RestServlet):
 
     def __init__(self, hs: "HomeServer"):
         super().__init__()
+        self._hs = hs
         self.clock = hs.get_clock()
         self.room_context_handler = hs.get_room_context_handler()
         self._event_serializer = hs.get_event_client_serializer()
@@ -688,7 +692,9 @@ class RoomEventContextServlet(RestServlet):
         filter_str = parse_string(request, "filter", encoding="utf-8")
         if filter_str:
             filter_json = urlparse.unquote(filter_str)
-            event_filter: Optional[Filter] = Filter(json_decoder.decode(filter_json))
+            event_filter: Optional[Filter] = Filter(
+                self._hs, json_decoder.decode(filter_json)
+            )
         else:
             event_filter = None
 
diff --git a/synapse/rest/client/sync.py b/synapse/rest/client/sync.py
index 913216a7c4..8c0fdb1940 100644
--- a/synapse/rest/client/sync.py
+++ b/synapse/rest/client/sync.py
@@ -29,7 +29,7 @@ from typing import (
 
 from synapse.api.constants import Membership, PresenceState
 from synapse.api.errors import Codes, StoreError, SynapseError
-from synapse.api.filtering import DEFAULT_FILTER_COLLECTION, FilterCollection
+from synapse.api.filtering import FilterCollection
 from synapse.api.presence import UserPresenceState
 from synapse.events import EventBase
 from synapse.events.utils import (
@@ -150,7 +150,7 @@ class SyncRestServlet(RestServlet):
         request_key = (user, timeout, since, filter_id, full_state, device_id)
 
         if filter_id is None:
-            filter_collection = DEFAULT_FILTER_COLLECTION
+            filter_collection = self.filtering.DEFAULT_FILTER_COLLECTION
         elif filter_id.startswith("{"):
             try:
                 filter_object = json_decoder.decode(filter_id)
@@ -160,7 +160,7 @@ class SyncRestServlet(RestServlet):
             except Exception:
                 raise SynapseError(400, "Invalid filter JSON")
             self.filtering.check_valid_filter(filter_object)
-            filter_collection = FilterCollection(filter_object)
+            filter_collection = FilterCollection(self.hs, filter_object)
         else:
             try:
                 filter_collection = await self.filtering.get_user_filter(
diff --git a/synapse/storage/databases/main/relations.py b/synapse/storage/databases/main/relations.py
index 53576ad52f..907af10995 100644
--- a/synapse/storage/databases/main/relations.py
+++ b/synapse/storage/databases/main/relations.py
@@ -20,7 +20,7 @@ import attr
 from synapse.api.constants import RelationTypes
 from synapse.events import EventBase
 from synapse.storage._base import SQLBaseStore
-from synapse.storage.database import LoggingTransaction
+from synapse.storage.database import LoggingTransaction, make_in_list_sql_clause
 from synapse.storage.databases.main.stream import generate_pagination_where_clause
 from synapse.storage.relations import (
     AggregationPaginationToken,
@@ -334,6 +334,62 @@ class RelationsWorkerStore(SQLBaseStore):
 
         return count, latest_event
 
+    async def events_have_relations(
+        self,
+        parent_ids: List[str],
+        relation_senders: Optional[List[str]],
+        relation_types: Optional[List[str]],
+    ) -> List[str]:
+        """Check which events have a relationship from the given senders of the
+        given types.
+
+        Args:
+            parent_ids: The events being annotated
+            relation_senders: The relation senders to check.
+            relation_types: The relation types to check.
+
+        Returns:
+            True if the event has at least one relationship from one of the given senders of the given type.
+        """
+        # If no restrictions are given then the event has the required relations.
+        if not relation_senders and not relation_types:
+            return parent_ids
+
+        sql = """
+            SELECT relates_to_id FROM event_relations
+            INNER JOIN events USING (event_id)
+            WHERE
+                %s;
+        """
+
+        def _get_if_event_has_relations(txn) -> List[str]:
+            clauses: List[str] = []
+            clause, args = make_in_list_sql_clause(
+                txn.database_engine, "relates_to_id", parent_ids
+            )
+            clauses.append(clause)
+
+            if relation_senders:
+                clause, temp_args = make_in_list_sql_clause(
+                    txn.database_engine, "sender", relation_senders
+                )
+                clauses.append(clause)
+                args.extend(temp_args)
+            if relation_types:
+                clause, temp_args = make_in_list_sql_clause(
+                    txn.database_engine, "relation_type", relation_types
+                )
+                clauses.append(clause)
+                args.extend(temp_args)
+
+            txn.execute(sql % " AND ".join(clauses), args)
+
+            return [row[0] for row in txn]
+
+        return await self.db_pool.runInteraction(
+            "get_if_event_has_relations", _get_if_event_has_relations
+        )
+
     async def has_user_annotated_event(
         self, parent_id: str, event_type: str, aggregation_key: str, sender: str
     ) -> bool:
diff --git a/synapse/storage/databases/main/stream.py b/synapse/storage/databases/main/stream.py
index dc7884b1c0..42dc807d17 100644
--- a/synapse/storage/databases/main/stream.py
+++ b/synapse/storage/databases/main/stream.py
@@ -272,31 +272,37 @@ def filter_to_clause(event_filter: Optional[Filter]) -> Tuple[str, List[str]]:
     args = []
 
     if event_filter.types:
-        clauses.append("(%s)" % " OR ".join("type = ?" for _ in event_filter.types))
+        clauses.append(
+            "(%s)" % " OR ".join("event.type = ?" for _ in event_filter.types)
+        )
         args.extend(event_filter.types)
 
     for typ in event_filter.not_types:
-        clauses.append("type != ?")
+        clauses.append("event.type != ?")
         args.append(typ)
 
     if event_filter.senders:
-        clauses.append("(%s)" % " OR ".join("sender = ?" for _ in event_filter.senders))
+        clauses.append(
+            "(%s)" % " OR ".join("event.sender = ?" for _ in event_filter.senders)
+        )
         args.extend(event_filter.senders)
 
     for sender in event_filter.not_senders:
-        clauses.append("sender != ?")
+        clauses.append("event.sender != ?")
         args.append(sender)
 
     if event_filter.rooms:
-        clauses.append("(%s)" % " OR ".join("room_id = ?" for _ in event_filter.rooms))
+        clauses.append(
+            "(%s)" % " OR ".join("event.room_id = ?" for _ in event_filter.rooms)
+        )
         args.extend(event_filter.rooms)
 
     for room_id in event_filter.not_rooms:
-        clauses.append("room_id != ?")
+        clauses.append("event.room_id != ?")
         args.append(room_id)
 
     if event_filter.contains_url:
-        clauses.append("contains_url = ?")
+        clauses.append("event.contains_url = ?")
         args.append(event_filter.contains_url)
 
     # We're only applying the "labels" filter on the database query, because applying the
@@ -307,6 +313,23 @@ def filter_to_clause(event_filter: Optional[Filter]) -> Tuple[str, List[str]]:
         clauses.append("(%s)" % " OR ".join("label = ?" for _ in event_filter.labels))
         args.extend(event_filter.labels)
 
+    # Filter on relation_senders / relation types from the joined tables.
+    if event_filter.relation_senders:
+        clauses.append(
+            "(%s)"
+            % " OR ".join(
+                "related_event.sender = ?" for _ in event_filter.relation_senders
+            )
+        )
+        args.extend(event_filter.relation_senders)
+
+    if event_filter.relation_types:
+        clauses.append(
+            "(%s)"
+            % " OR ".join("relation_type = ?" for _ in event_filter.relation_types)
+        )
+        args.extend(event_filter.relation_types)
+
     return " AND ".join(clauses), args
 
 
@@ -1116,7 +1139,7 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore, metaclass=abc.ABCMeta):
 
         bounds = generate_pagination_where_clause(
             direction=direction,
-            column_names=("topological_ordering", "stream_ordering"),
+            column_names=("event.topological_ordering", "event.stream_ordering"),
             from_token=from_bound,
             to_token=to_bound,
             engine=self.database_engine,
@@ -1133,32 +1156,51 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore, metaclass=abc.ABCMeta):
 
         select_keywords = "SELECT"
         join_clause = ""
+        # Using DISTINCT in this SELECT query is quite expensive, because it
+        # requires the engine to sort on the entire (not limited) result set,
+        # i.e. the entire events table. Only use it in scenarios that could result
+        # in the same event ID occurring multiple times in the results.
+        needs_distinct = False
         if event_filter and event_filter.labels:
             # If we're not filtering on a label, then joining on event_labels will
             # return as many row for a single event as the number of labels it has. To
             # avoid this, only join if we're filtering on at least one label.
-            join_clause = """
+            join_clause += """
                 LEFT JOIN event_labels
                 USING (event_id, room_id, topological_ordering)
             """
             if len(event_filter.labels) > 1:
-                # Using DISTINCT in this SELECT query is quite expensive, because it
-                # requires the engine to sort on the entire (not limited) result set,
-                # i.e. the entire events table. We only need to use it when we're
-                # filtering on more than two labels, because that's the only scenario
-                # in which we can possibly to get multiple times the same event ID in
-                # the results.
-                select_keywords += "DISTINCT"
+                # Multiple labels could cause the same event to appear multiple times.
+                needs_distinct = True
+
+        # If there is a filter on relation_senders and relation_types join to the
+        # relations table.
+        if event_filter and (
+            event_filter.relation_senders or event_filter.relation_types
+        ):
+            # Filtering by relations could cause the same event to appear multiple
+            # times (since there's no limit on the number of relations to an event).
+            needs_distinct = True
+            join_clause += """
+                LEFT JOIN event_relations AS relation ON (event.event_id = relation.relates_to_id)
+            """
+            if event_filter.relation_senders:
+                join_clause += """
+                    LEFT JOIN events AS related_event ON (relation.event_id = related_event.event_id)
+                """
+
+        if needs_distinct:
+            select_keywords += " DISTINCT"
 
         sql = """
             %(select_keywords)s
-                event_id, instance_name,
-                topological_ordering, stream_ordering
-            FROM events
+                event.event_id, event.instance_name,
+                event.topological_ordering, event.stream_ordering
+            FROM events AS event
             %(join_clause)s
-            WHERE outlier = ? AND room_id = ? AND %(bounds)s
-            ORDER BY topological_ordering %(order)s,
-            stream_ordering %(order)s LIMIT ?
+            WHERE event.outlier = ? AND event.room_id = ? AND %(bounds)s
+            ORDER BY event.topological_ordering %(order)s,
+            event.stream_ordering %(order)s LIMIT ?
         """ % {
             "select_keywords": select_keywords,
             "join_clause": join_clause,
diff --git a/tests/api/test_filtering.py b/tests/api/test_filtering.py
index f44c91a373..b7fc33dc94 100644
--- a/tests/api/test_filtering.py
+++ b/tests/api/test_filtering.py
@@ -15,6 +15,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from unittest.mock import patch
+
 import jsonschema
 
 from synapse.api.constants import EventContentFields
@@ -51,9 +53,8 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             {"presence": {"senders": ["@bar;pik.test.com"]}},
         ]
         for filter in invalid_filters:
-            with self.assertRaises(SynapseError) as check_filter_error:
+            with self.assertRaises(SynapseError):
                 self.filtering.check_valid_filter(filter)
-                self.assertIsInstance(check_filter_error.exception, SynapseError)
 
     def test_valid_filters(self):
         valid_filters = [
@@ -119,12 +120,12 @@ class FilteringTestCase(unittest.HomeserverTestCase):
         definition = {"types": ["m.room.message", "org.matrix.foo.bar"]}
         event = MockEvent(sender="@foo:bar", type="m.room.message", room_id="!foo:bar")
 
-        self.assertTrue(Filter(definition).check(event))
+        self.assertTrue(Filter(self.hs, definition)._check(event))
 
     def test_definition_types_works_with_wildcards(self):
         definition = {"types": ["m.*", "org.matrix.foo.bar"]}
         event = MockEvent(sender="@foo:bar", type="m.room.message", room_id="!foo:bar")
-        self.assertTrue(Filter(definition).check(event))
+        self.assertTrue(Filter(self.hs, definition)._check(event))
 
     def test_definition_types_works_with_unknowns(self):
         definition = {"types": ["m.room.message", "org.matrix.foo.bar"]}
@@ -133,24 +134,24 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             type="now.for.something.completely.different",
             room_id="!foo:bar",
         )
-        self.assertFalse(Filter(definition).check(event))
+        self.assertFalse(Filter(self.hs, definition)._check(event))
 
     def test_definition_not_types_works_with_literals(self):
         definition = {"not_types": ["m.room.message", "org.matrix.foo.bar"]}
         event = MockEvent(sender="@foo:bar", type="m.room.message", room_id="!foo:bar")
-        self.assertFalse(Filter(definition).check(event))
+        self.assertFalse(Filter(self.hs, definition)._check(event))
 
     def test_definition_not_types_works_with_wildcards(self):
         definition = {"not_types": ["m.room.message", "org.matrix.*"]}
         event = MockEvent(
             sender="@foo:bar", type="org.matrix.custom.event", room_id="!foo:bar"
         )
-        self.assertFalse(Filter(definition).check(event))
+        self.assertFalse(Filter(self.hs, definition)._check(event))
 
     def test_definition_not_types_works_with_unknowns(self):
         definition = {"not_types": ["m.*", "org.*"]}
         event = MockEvent(sender="@foo:bar", type="com.nom.nom.nom", room_id="!foo:bar")
-        self.assertTrue(Filter(definition).check(event))
+        self.assertTrue(Filter(self.hs, definition)._check(event))
 
     def test_definition_not_types_takes_priority_over_types(self):
         definition = {
@@ -158,35 +159,35 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             "types": ["m.room.message", "m.room.topic"],
         }
         event = MockEvent(sender="@foo:bar", type="m.room.topic", room_id="!foo:bar")
-        self.assertFalse(Filter(definition).check(event))
+        self.assertFalse(Filter(self.hs, definition)._check(event))
 
     def test_definition_senders_works_with_literals(self):
         definition = {"senders": ["@flibble:wibble"]}
         event = MockEvent(
             sender="@flibble:wibble", type="com.nom.nom.nom", room_id="!foo:bar"
         )
-        self.assertTrue(Filter(definition).check(event))
+        self.assertTrue(Filter(self.hs, definition)._check(event))
 
     def test_definition_senders_works_with_unknowns(self):
         definition = {"senders": ["@flibble:wibble"]}
         event = MockEvent(
             sender="@challenger:appears", type="com.nom.nom.nom", room_id="!foo:bar"
         )
-        self.assertFalse(Filter(definition).check(event))
+        self.assertFalse(Filter(self.hs, definition)._check(event))
 
     def test_definition_not_senders_works_with_literals(self):
         definition = {"not_senders": ["@flibble:wibble"]}
         event = MockEvent(
             sender="@flibble:wibble", type="com.nom.nom.nom", room_id="!foo:bar"
         )
-        self.assertFalse(Filter(definition).check(event))
+        self.assertFalse(Filter(self.hs, definition)._check(event))
 
     def test_definition_not_senders_works_with_unknowns(self):
         definition = {"not_senders": ["@flibble:wibble"]}
         event = MockEvent(
             sender="@challenger:appears", type="com.nom.nom.nom", room_id="!foo:bar"
         )
-        self.assertTrue(Filter(definition).check(event))
+        self.assertTrue(Filter(self.hs, definition)._check(event))
 
     def test_definition_not_senders_takes_priority_over_senders(self):
         definition = {
@@ -196,14 +197,14 @@ class FilteringTestCase(unittest.HomeserverTestCase):
         event = MockEvent(
             sender="@misspiggy:muppets", type="m.room.topic", room_id="!foo:bar"
         )
-        self.assertFalse(Filter(definition).check(event))
+        self.assertFalse(Filter(self.hs, definition)._check(event))
 
     def test_definition_rooms_works_with_literals(self):
         definition = {"rooms": ["!secretbase:unknown"]}
         event = MockEvent(
             sender="@foo:bar", type="m.room.message", room_id="!secretbase:unknown"
         )
-        self.assertTrue(Filter(definition).check(event))
+        self.assertTrue(Filter(self.hs, definition)._check(event))
 
     def test_definition_rooms_works_with_unknowns(self):
         definition = {"rooms": ["!secretbase:unknown"]}
@@ -212,7 +213,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             type="m.room.message",
             room_id="!anothersecretbase:unknown",
         )
-        self.assertFalse(Filter(definition).check(event))
+        self.assertFalse(Filter(self.hs, definition)._check(event))
 
     def test_definition_not_rooms_works_with_literals(self):
         definition = {"not_rooms": ["!anothersecretbase:unknown"]}
@@ -221,7 +222,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             type="m.room.message",
             room_id="!anothersecretbase:unknown",
         )
-        self.assertFalse(Filter(definition).check(event))
+        self.assertFalse(Filter(self.hs, definition)._check(event))
 
     def test_definition_not_rooms_works_with_unknowns(self):
         definition = {"not_rooms": ["!secretbase:unknown"]}
@@ -230,7 +231,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             type="m.room.message",
             room_id="!anothersecretbase:unknown",
         )
-        self.assertTrue(Filter(definition).check(event))
+        self.assertTrue(Filter(self.hs, definition)._check(event))
 
     def test_definition_not_rooms_takes_priority_over_rooms(self):
         definition = {
@@ -240,7 +241,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
         event = MockEvent(
             sender="@foo:bar", type="m.room.message", room_id="!secretbase:unknown"
         )
-        self.assertFalse(Filter(definition).check(event))
+        self.assertFalse(Filter(self.hs, definition)._check(event))
 
     def test_definition_combined_event(self):
         definition = {
@@ -256,7 +257,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             type="m.room.message",  # yup
             room_id="!stage:unknown",  # yup
         )
-        self.assertTrue(Filter(definition).check(event))
+        self.assertTrue(Filter(self.hs, definition)._check(event))
 
     def test_definition_combined_event_bad_sender(self):
         definition = {
@@ -272,7 +273,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             type="m.room.message",  # yup
             room_id="!stage:unknown",  # yup
         )
-        self.assertFalse(Filter(definition).check(event))
+        self.assertFalse(Filter(self.hs, definition)._check(event))
 
     def test_definition_combined_event_bad_room(self):
         definition = {
@@ -288,7 +289,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             type="m.room.message",  # yup
             room_id="!piggyshouse:muppets",  # nope
         )
-        self.assertFalse(Filter(definition).check(event))
+        self.assertFalse(Filter(self.hs, definition)._check(event))
 
     def test_definition_combined_event_bad_type(self):
         definition = {
@@ -304,7 +305,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             type="muppets.misspiggy.kisses",  # nope
             room_id="!stage:unknown",  # yup
         )
-        self.assertFalse(Filter(definition).check(event))
+        self.assertFalse(Filter(self.hs, definition)._check(event))
 
     def test_filter_labels(self):
         definition = {"org.matrix.labels": ["#fun"]}
@@ -315,7 +316,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             content={EventContentFields.LABELS: ["#fun"]},
         )
 
-        self.assertTrue(Filter(definition).check(event))
+        self.assertTrue(Filter(self.hs, definition)._check(event))
 
         event = MockEvent(
             sender="@foo:bar",
@@ -324,7 +325,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             content={EventContentFields.LABELS: ["#notfun"]},
         )
 
-        self.assertFalse(Filter(definition).check(event))
+        self.assertFalse(Filter(self.hs, definition)._check(event))
 
     def test_filter_not_labels(self):
         definition = {"org.matrix.not_labels": ["#fun"]}
@@ -335,7 +336,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             content={EventContentFields.LABELS: ["#fun"]},
         )
 
-        self.assertFalse(Filter(definition).check(event))
+        self.assertFalse(Filter(self.hs, definition)._check(event))
 
         event = MockEvent(
             sender="@foo:bar",
@@ -344,7 +345,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             content={EventContentFields.LABELS: ["#notfun"]},
         )
 
-        self.assertTrue(Filter(definition).check(event))
+        self.assertTrue(Filter(self.hs, definition)._check(event))
 
     def test_filter_presence_match(self):
         user_filter_json = {"presence": {"types": ["m.*"]}}
@@ -362,7 +363,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             )
         )
 
-        results = user_filter.filter_presence(events=events)
+        results = self.get_success(user_filter.filter_presence(events=events))
         self.assertEquals(events, results)
 
     def test_filter_presence_no_match(self):
@@ -386,7 +387,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             )
         )
 
-        results = user_filter.filter_presence(events=events)
+        results = self.get_success(user_filter.filter_presence(events=events))
         self.assertEquals([], results)
 
     def test_filter_room_state_match(self):
@@ -405,7 +406,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             )
         )
 
-        results = user_filter.filter_room_state(events=events)
+        results = self.get_success(user_filter.filter_room_state(events=events))
         self.assertEquals(events, results)
 
     def test_filter_room_state_no_match(self):
@@ -426,7 +427,7 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             )
         )
 
-        results = user_filter.filter_room_state(events)
+        results = self.get_success(user_filter.filter_room_state(events))
         self.assertEquals([], results)
 
     def test_filter_rooms(self):
@@ -441,10 +442,52 @@ class FilteringTestCase(unittest.HomeserverTestCase):
             "!not_included:example.com",  # Disallowed because not in rooms.
         ]
 
-        filtered_room_ids = list(Filter(definition).filter_rooms(room_ids))
+        filtered_room_ids = list(Filter(self.hs, definition).filter_rooms(room_ids))
 
         self.assertEquals(filtered_room_ids, ["!allowed:example.com"])
 
+    @unittest.override_config({"experimental_features": {"msc3440_enabled": True}})
+    def test_filter_relations(self):
+        events = [
+            # An event without a relation.
+            MockEvent(
+                event_id="$no_relation",
+                sender="@foo:bar",
+                type="org.matrix.custom.event",
+                room_id="!foo:bar",
+            ),
+            # An event with a relation.
+            MockEvent(
+                event_id="$with_relation",
+                sender="@foo:bar",
+                type="org.matrix.custom.event",
+                room_id="!foo:bar",
+            ),
+            # Non-EventBase objects get passed through.
+            {},
+        ]
+
+        # For the following tests we patch the datastore method (intead of injecting
+        # events). This is a bit cheeky, but tests the logic of _check_event_relations.
+
+        # Filter for a particular sender.
+        definition = {
+            "io.element.relation_senders": ["@foo:bar"],
+        }
+
+        async def events_have_relations(*args, **kwargs):
+            return ["$with_relation"]
+
+        with patch.object(
+            self.datastore, "events_have_relations", new=events_have_relations
+        ):
+            filtered_events = list(
+                self.get_success(
+                    Filter(self.hs, definition)._check_event_relations(events)
+                )
+            )
+        self.assertEquals(filtered_events, events[1:])
+
     def test_add_filter(self):
         user_filter_json = {"room": {"state": {"types": ["m.*"]}}}
 
diff --git a/tests/handlers/test_sync.py b/tests/handlers/test_sync.py
index 339c039914..638186f173 100644
--- a/tests/handlers/test_sync.py
+++ b/tests/handlers/test_sync.py
@@ -13,10 +13,11 @@
 # limitations under the License.
 
 from typing import Optional
+from unittest.mock import Mock
 
 from synapse.api.constants import EventTypes, JoinRules
 from synapse.api.errors import Codes, ResourceLimitError
-from synapse.api.filtering import DEFAULT_FILTER_COLLECTION
+from synapse.api.filtering import Filtering
 from synapse.api.room_versions import RoomVersions
 from synapse.handlers.sync import SyncConfig
 from synapse.rest import admin
@@ -197,7 +198,7 @@ def generate_sync_config(
     _request_key += 1
     return SyncConfig(
         user=UserID.from_string(user_id),
-        filter_collection=DEFAULT_FILTER_COLLECTION,
+        filter_collection=Filtering(Mock()).DEFAULT_FILTER_COLLECTION,
         is_guest=False,
         request_key=("request_key", _request_key),
         device_id=device_id,
diff --git a/tests/rest/client/test_rooms.py b/tests/rest/client/test_rooms.py
index 376853fd65..10a4a4dc5e 100644
--- a/tests/rest/client/test_rooms.py
+++ b/tests/rest/client/test_rooms.py
@@ -25,7 +25,12 @@ from urllib import parse as urlparse
 from twisted.internet import defer
 
 import synapse.rest.admin
-from synapse.api.constants import EventContentFields, EventTypes, Membership
+from synapse.api.constants import (
+    EventContentFields,
+    EventTypes,
+    Membership,
+    RelationTypes,
+)
 from synapse.api.errors import Codes, HttpResponseException
 from synapse.handlers.pagination import PurgeStatus
 from synapse.rest import admin
@@ -2157,6 +2162,153 @@ class LabelsTestCase(unittest.HomeserverTestCase):
         return event_id
 
 
+class RelationsTestCase(unittest.HomeserverTestCase):
+    servlets = [
+        synapse.rest.admin.register_servlets_for_client_rest_resource,
+        room.register_servlets,
+        login.register_servlets,
+    ]
+
+    def default_config(self):
+        config = super().default_config()
+        config["experimental_features"] = {"msc3440_enabled": True}
+        return config
+
+    def prepare(self, reactor, clock, homeserver):
+        self.user_id = self.register_user("test", "test")
+        self.tok = self.login("test", "test")
+        self.room_id = self.helper.create_room_as(self.user_id, tok=self.tok)
+
+        self.second_user_id = self.register_user("second", "test")
+        self.second_tok = self.login("second", "test")
+        self.helper.join(
+            room=self.room_id, user=self.second_user_id, tok=self.second_tok
+        )
+
+        self.third_user_id = self.register_user("third", "test")
+        self.third_tok = self.login("third", "test")
+        self.helper.join(room=self.room_id, user=self.third_user_id, tok=self.third_tok)
+
+        # An initial event with a relation from second user.
+        res = self.helper.send_event(
+            room_id=self.room_id,
+            type=EventTypes.Message,
+            content={"msgtype": "m.text", "body": "Message 1"},
+            tok=self.tok,
+        )
+        self.event_id_1 = res["event_id"]
+        self.helper.send_event(
+            room_id=self.room_id,
+            type="m.reaction",
+            content={
+                "m.relates_to": {
+                    "rel_type": RelationTypes.ANNOTATION,
+                    "event_id": self.event_id_1,
+                    "key": "👍",
+                }
+            },
+            tok=self.second_tok,
+        )
+
+        # Another event with a relation from third user.
+        res = self.helper.send_event(
+            room_id=self.room_id,
+            type=EventTypes.Message,
+            content={"msgtype": "m.text", "body": "Message 2"},
+            tok=self.tok,
+        )
+        self.event_id_2 = res["event_id"]
+        self.helper.send_event(
+            room_id=self.room_id,
+            type="m.reaction",
+            content={
+                "m.relates_to": {
+                    "rel_type": RelationTypes.REFERENCE,
+                    "event_id": self.event_id_2,
+                }
+            },
+            tok=self.third_tok,
+        )
+
+        # An event with no relations.
+        self.helper.send_event(
+            room_id=self.room_id,
+            type=EventTypes.Message,
+            content={"msgtype": "m.text", "body": "No relations"},
+            tok=self.tok,
+        )
+
+    def _filter_messages(self, filter: JsonDict) -> List[JsonDict]:
+        """Make a request to /messages with a filter, returns the chunk of events."""
+        channel = self.make_request(
+            "GET",
+            "/rooms/%s/messages?filter=%s&dir=b" % (self.room_id, json.dumps(filter)),
+            access_token=self.tok,
+        )
+        self.assertEqual(channel.code, 200, channel.result)
+
+        return channel.json_body["chunk"]
+
+    def test_filter_relation_senders(self):
+        # Messages which second user reacted to.
+        filter = {"io.element.relation_senders": [self.second_user_id]}
+        chunk = self._filter_messages(filter)
+        self.assertEqual(len(chunk), 1, chunk)
+        self.assertEqual(chunk[0]["event_id"], self.event_id_1)
+
+        # Messages which third user reacted to.
+        filter = {"io.element.relation_senders": [self.third_user_id]}
+        chunk = self._filter_messages(filter)
+        self.assertEqual(len(chunk), 1, chunk)
+        self.assertEqual(chunk[0]["event_id"], self.event_id_2)
+
+        # Messages which either user reacted to.
+        filter = {
+            "io.element.relation_senders": [self.second_user_id, self.third_user_id]
+        }
+        chunk = self._filter_messages(filter)
+        self.assertEqual(len(chunk), 2, chunk)
+        self.assertCountEqual(
+            [c["event_id"] for c in chunk], [self.event_id_1, self.event_id_2]
+        )
+
+    def test_filter_relation_type(self):
+        # Messages which have annotations.
+        filter = {"io.element.relation_types": [RelationTypes.ANNOTATION]}
+        chunk = self._filter_messages(filter)
+        self.assertEqual(len(chunk), 1, chunk)
+        self.assertEqual(chunk[0]["event_id"], self.event_id_1)
+
+        # Messages which have references.
+        filter = {"io.element.relation_types": [RelationTypes.REFERENCE]}
+        chunk = self._filter_messages(filter)
+        self.assertEqual(len(chunk), 1, chunk)
+        self.assertEqual(chunk[0]["event_id"], self.event_id_2)
+
+        # Messages which have either annotations or references.
+        filter = {
+            "io.element.relation_types": [
+                RelationTypes.ANNOTATION,
+                RelationTypes.REFERENCE,
+            ]
+        }
+        chunk = self._filter_messages(filter)
+        self.assertEqual(len(chunk), 2, chunk)
+        self.assertCountEqual(
+            [c["event_id"] for c in chunk], [self.event_id_1, self.event_id_2]
+        )
+
+    def test_filter_relation_senders_and_type(self):
+        # Messages which second user reacted to.
+        filter = {
+            "io.element.relation_senders": [self.second_user_id],
+            "io.element.relation_types": [RelationTypes.ANNOTATION],
+        }
+        chunk = self._filter_messages(filter)
+        self.assertEqual(len(chunk), 1, chunk)
+        self.assertEqual(chunk[0]["event_id"], self.event_id_1)
+
+
 class ContextTestCase(unittest.HomeserverTestCase):
 
     servlets = [
diff --git a/tests/storage/test_stream.py b/tests/storage/test_stream.py
new file mode 100644
index 0000000000..ce782c7e1d
--- /dev/null
+++ b/tests/storage/test_stream.py
@@ -0,0 +1,207 @@
+#  Copyright 2021 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 typing import List
+
+from synapse.api.constants import EventTypes, RelationTypes
+from synapse.api.filtering import Filter
+from synapse.events import EventBase
+from synapse.rest import admin
+from synapse.rest.client import login, room
+from synapse.types import JsonDict
+
+from tests.unittest import HomeserverTestCase
+
+
+class PaginationTestCase(HomeserverTestCase):
+    """
+    Test the pre-filtering done in the pagination code.
+
+    This is similar to some of the tests in tests.rest.client.test_rooms but here
+    we ensure that the filtering done in the database is applied successfully.
+    """
+
+    servlets = [
+        admin.register_servlets_for_client_rest_resource,
+        room.register_servlets,
+        login.register_servlets,
+    ]
+
+    def default_config(self):
+        config = super().default_config()
+        config["experimental_features"] = {"msc3440_enabled": True}
+        return config
+
+    def prepare(self, reactor, clock, homeserver):
+        self.user_id = self.register_user("test", "test")
+        self.tok = self.login("test", "test")
+        self.room_id = self.helper.create_room_as(self.user_id, tok=self.tok)
+
+        self.second_user_id = self.register_user("second", "test")
+        self.second_tok = self.login("second", "test")
+        self.helper.join(
+            room=self.room_id, user=self.second_user_id, tok=self.second_tok
+        )
+
+        self.third_user_id = self.register_user("third", "test")
+        self.third_tok = self.login("third", "test")
+        self.helper.join(room=self.room_id, user=self.third_user_id, tok=self.third_tok)
+
+        # An initial event with a relation from second user.
+        res = self.helper.send_event(
+            room_id=self.room_id,
+            type=EventTypes.Message,
+            content={"msgtype": "m.text", "body": "Message 1"},
+            tok=self.tok,
+        )
+        self.event_id_1 = res["event_id"]
+        self.helper.send_event(
+            room_id=self.room_id,
+            type="m.reaction",
+            content={
+                "m.relates_to": {
+                    "rel_type": RelationTypes.ANNOTATION,
+                    "event_id": self.event_id_1,
+                    "key": "👍",
+                }
+            },
+            tok=self.second_tok,
+        )
+
+        # Another event with a relation from third user.
+        res = self.helper.send_event(
+            room_id=self.room_id,
+            type=EventTypes.Message,
+            content={"msgtype": "m.text", "body": "Message 2"},
+            tok=self.tok,
+        )
+        self.event_id_2 = res["event_id"]
+        self.helper.send_event(
+            room_id=self.room_id,
+            type="m.reaction",
+            content={
+                "m.relates_to": {
+                    "rel_type": RelationTypes.REFERENCE,
+                    "event_id": self.event_id_2,
+                }
+            },
+            tok=self.third_tok,
+        )
+
+        # An event with no relations.
+        self.helper.send_event(
+            room_id=self.room_id,
+            type=EventTypes.Message,
+            content={"msgtype": "m.text", "body": "No relations"},
+            tok=self.tok,
+        )
+
+    def _filter_messages(self, filter: JsonDict) -> List[EventBase]:
+        """Make a request to /messages with a filter, returns the chunk of events."""
+
+        from_token = self.get_success(
+            self.hs.get_event_sources().get_current_token_for_pagination()
+        )
+
+        events, next_key = self.get_success(
+            self.hs.get_datastore().paginate_room_events(
+                room_id=self.room_id,
+                from_key=from_token.room_key,
+                to_key=None,
+                direction="b",
+                limit=10,
+                event_filter=Filter(self.hs, filter),
+            )
+        )
+
+        return events
+
+    def test_filter_relation_senders(self):
+        # Messages which second user reacted to.
+        filter = {"io.element.relation_senders": [self.second_user_id]}
+        chunk = self._filter_messages(filter)
+        self.assertEqual(len(chunk), 1, chunk)
+        self.assertEqual(chunk[0].event_id, self.event_id_1)
+
+        # Messages which third user reacted to.
+        filter = {"io.element.relation_senders": [self.third_user_id]}
+        chunk = self._filter_messages(filter)
+        self.assertEqual(len(chunk), 1, chunk)
+        self.assertEqual(chunk[0].event_id, self.event_id_2)
+
+        # Messages which either user reacted to.
+        filter = {
+            "io.element.relation_senders": [self.second_user_id, self.third_user_id]
+        }
+        chunk = self._filter_messages(filter)
+        self.assertEqual(len(chunk), 2, chunk)
+        self.assertCountEqual(
+            [c.event_id for c in chunk], [self.event_id_1, self.event_id_2]
+        )
+
+    def test_filter_relation_type(self):
+        # Messages which have annotations.
+        filter = {"io.element.relation_types": [RelationTypes.ANNOTATION]}
+        chunk = self._filter_messages(filter)
+        self.assertEqual(len(chunk), 1, chunk)
+        self.assertEqual(chunk[0].event_id, self.event_id_1)
+
+        # Messages which have references.
+        filter = {"io.element.relation_types": [RelationTypes.REFERENCE]}
+        chunk = self._filter_messages(filter)
+        self.assertEqual(len(chunk), 1, chunk)
+        self.assertEqual(chunk[0].event_id, self.event_id_2)
+
+        # Messages which have either annotations or references.
+        filter = {
+            "io.element.relation_types": [
+                RelationTypes.ANNOTATION,
+                RelationTypes.REFERENCE,
+            ]
+        }
+        chunk = self._filter_messages(filter)
+        self.assertEqual(len(chunk), 2, chunk)
+        self.assertCountEqual(
+            [c.event_id for c in chunk], [self.event_id_1, self.event_id_2]
+        )
+
+    def test_filter_relation_senders_and_type(self):
+        # Messages which second user reacted to.
+        filter = {
+            "io.element.relation_senders": [self.second_user_id],
+            "io.element.relation_types": [RelationTypes.ANNOTATION],
+        }
+        chunk = self._filter_messages(filter)
+        self.assertEqual(len(chunk), 1, chunk)
+        self.assertEqual(chunk[0].event_id, self.event_id_1)
+
+    def test_duplicate_relation(self):
+        """An event should only be returned once if there are multiple relations to it."""
+        self.helper.send_event(
+            room_id=self.room_id,
+            type="m.reaction",
+            content={
+                "m.relates_to": {
+                    "rel_type": RelationTypes.ANNOTATION,
+                    "event_id": self.event_id_1,
+                    "key": "A",
+                }
+            },
+            tok=self.second_tok,
+        )
+
+        filter = {"io.element.relation_senders": [self.second_user_id]}
+        chunk = self._filter_messages(filter)
+        self.assertEqual(len(chunk), 1, chunk)
+        self.assertEqual(chunk[0].event_id, self.event_id_1)