1 files changed, 50 insertions, 0 deletions
diff --git a/synapse/rest/client/v2_alpha/relations.py b/synapse/rest/client/v2_alpha/relations.py
index b504b4a8be..bac9e85c21 100644
--- a/synapse/rest/client/v2_alpha/relations.py
+++ b/synapse/rest/client/v2_alpha/relations.py
@@ -27,6 +27,7 @@ from synapse.api.constants import EventTypes
 from synapse.api.errors import SynapseError
 from synapse.http.servlet import (
     RestServlet,
+    parse_integer,
     parse_json_object_from_request,
     parse_string,
 )
@@ -106,5 +107,54 @@ class RelationSendServlet(RestServlet):
         defer.returnValue((200, {"event_id": event.event_id}))
 
 
+class RelationPaginationServlet(RestServlet):
+    """API to paginate relations on an event by topological ordering, optionally
+    filtered by relation type and event type.
+    """
+
+    PATTERNS = client_v2_patterns(
+        "/rooms/(?P<room_id>[^/]*)/relations/(?P<parent_id>[^/]*)"
+        "(/(?P<relation_type>[^/]*)(/(?P<event_type>[^/]*))?)?$",
+        releases=(),
+    )
+
+    def __init__(self, hs):
+        super(RelationPaginationServlet, self).__init__()
+        self.auth = hs.get_auth()
+        self.store = hs.get_datastore()
+        self.clock = hs.get_clock()
+        self._event_serializer = hs.get_event_client_serializer()
+
+    @defer.inlineCallbacks
+    def on_GET(self, request, room_id, parent_id, relation_type=None, event_type=None):
+        requester = yield self.auth.get_user_by_req(request, allow_guest=True)
+
+        yield self.auth.check_in_room_or_world_readable(
+            room_id, requester.user.to_string()
+        )
+
+        limit = parse_integer(request, "limit", default=5)
+
+        result = yield self.store.get_relations_for_event(
+            event_id=parent_id,
+            relation_type=relation_type,
+            event_type=event_type,
+            limit=limit,
+        )
+
+        events = yield self.store.get_events_as_list(
+            [c["event_id"] for c in result.chunk]
+        )
+
+        now = self.clock.time_msec()
+        events = yield self._event_serializer.serialize_events(events, now)
+
+        return_value = result.to_dict()
+        return_value["chunk"] = events
+
+        defer.returnValue((200, return_value))
+
+
 def register_servlets(hs, http_server):
     RelationSendServlet(hs).register(http_server)
+    RelationPaginationServlet(hs).register(http_server)