diff --git a/synapse/handlers/relations.py b/synapse/handlers/relations.py
new file mode 100644
index 0000000000..57135d4519
--- /dev/null
+++ b/synapse/handlers/relations.py
@@ -0,0 +1,264 @@
+# 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.
+import logging
+from typing import TYPE_CHECKING, Dict, Iterable, Optional, cast
+
+import attr
+from frozendict import frozendict
+
+from synapse.api.constants import RelationTypes
+from synapse.api.errors import SynapseError
+from synapse.events import EventBase
+from synapse.types import JsonDict, Requester, StreamToken
+
+if TYPE_CHECKING:
+ from synapse.server import HomeServer
+ from synapse.storage.databases.main import DataStore
+
+
+logger = logging.getLogger(__name__)
+
+
+@attr.s(slots=True, frozen=True, auto_attribs=True)
+class _ThreadAggregation:
+ # The latest event in the thread.
+ latest_event: EventBase
+ # The latest edit to the latest event in the thread.
+ latest_edit: Optional[EventBase]
+ # The total number of events in the thread.
+ count: int
+ # True if the current user has sent an event to the thread.
+ current_user_participated: bool
+
+
+@attr.s(slots=True, auto_attribs=True)
+class BundledAggregations:
+ """
+ The bundled aggregations for an event.
+
+ Some values require additional processing during serialization.
+ """
+
+ annotations: Optional[JsonDict] = None
+ references: Optional[JsonDict] = None
+ replace: Optional[EventBase] = None
+ thread: Optional[_ThreadAggregation] = None
+
+ def __bool__(self) -> bool:
+ return bool(self.annotations or self.references or self.replace or self.thread)
+
+
+class RelationsHandler:
+ def __init__(self, hs: "HomeServer"):
+ self._main_store = hs.get_datastores().main
+ self._auth = hs.get_auth()
+ self._clock = hs.get_clock()
+ self._event_handler = hs.get_event_handler()
+ self._event_serializer = hs.get_event_client_serializer()
+
+ async def get_relations(
+ self,
+ requester: Requester,
+ event_id: str,
+ room_id: str,
+ relation_type: Optional[str] = None,
+ event_type: Optional[str] = None,
+ aggregation_key: Optional[str] = None,
+ limit: int = 5,
+ direction: str = "b",
+ from_token: Optional[StreamToken] = None,
+ to_token: Optional[StreamToken] = None,
+ ) -> JsonDict:
+ """Get related events of a event, ordered by topological ordering.
+
+ TODO Accept a PaginationConfig instead of individual pagination parameters.
+
+ Args:
+ requester: The user requesting the relations.
+ event_id: Fetch events that relate to this event ID.
+ room_id: The room the event belongs to.
+ relation_type: Only fetch events with this relation type, if given.
+ event_type: Only fetch events with this event type, if given.
+ aggregation_key: Only fetch events with this aggregation key, if given.
+ limit: Only fetch the most recent `limit` events.
+ direction: Whether to fetch the most recent first (`"b"`) or the
+ oldest first (`"f"`).
+ from_token: Fetch rows from the given token, or from the start if None.
+ to_token: Fetch rows up to the given token, or up to the end if None.
+
+ Returns:
+ The pagination chunk.
+ """
+
+ user_id = requester.user.to_string()
+
+ await self._auth.check_user_in_room_or_world_readable(
+ room_id, user_id, allow_departed_users=True
+ )
+
+ # This gets the original event and checks that a) the event exists and
+ # b) the user is allowed to view it.
+ event = await self._event_handler.get_event(requester.user, room_id, event_id)
+ if event is None:
+ raise SynapseError(404, "Unknown parent event.")
+
+ pagination_chunk = await self._main_store.get_relations_for_event(
+ event_id=event_id,
+ event=event,
+ room_id=room_id,
+ relation_type=relation_type,
+ event_type=event_type,
+ aggregation_key=aggregation_key,
+ limit=limit,
+ direction=direction,
+ from_token=from_token,
+ to_token=to_token,
+ )
+
+ events = await self._main_store.get_events_as_list(
+ [c["event_id"] for c in pagination_chunk.chunk]
+ )
+
+ now = self._clock.time_msec()
+ # Do not bundle aggregations when retrieving the original event because
+ # we want the content before relations are applied to it.
+ original_event = self._event_serializer.serialize_event(
+ event, now, bundle_aggregations=None
+ )
+ # The relations returned for the requested event do include their
+ # bundled aggregations.
+ aggregations = await self.get_bundled_aggregations(
+ events, requester.user.to_string()
+ )
+ serialized_events = self._event_serializer.serialize_events(
+ events, now, bundle_aggregations=aggregations
+ )
+
+ return_value = await pagination_chunk.to_dict(self._main_store)
+ return_value["chunk"] = serialized_events
+ return_value["original_event"] = original_event
+
+ return return_value
+
+ async def _get_bundled_aggregation_for_event(
+ self, event: EventBase, user_id: str
+ ) -> Optional[BundledAggregations]:
+ """Generate bundled aggregations for an event.
+
+ Note that this does not use a cache, but depends on cached methods.
+
+ Args:
+ event: The event to calculate bundled aggregations for.
+ user_id: The user requesting the bundled aggregations.
+
+ Returns:
+ The bundled aggregations for an event, if bundled aggregations are
+ enabled and the event can have bundled aggregations.
+ """
+
+ # Do not bundle aggregations for an event which represents an edit or an
+ # annotation. It does not make sense for them to have related events.
+ relates_to = event.content.get("m.relates_to")
+ if isinstance(relates_to, (dict, frozendict)):
+ relation_type = relates_to.get("rel_type")
+ if relation_type in (RelationTypes.ANNOTATION, RelationTypes.REPLACE):
+ return None
+
+ event_id = event.event_id
+ room_id = event.room_id
+
+ # The bundled aggregations to include, a mapping of relation type to a
+ # type-specific value. Some types include the direct return type here
+ # while others need more processing during serialization.
+ aggregations = BundledAggregations()
+
+ annotations = await self._main_store.get_aggregation_groups_for_event(
+ event_id, room_id
+ )
+ if annotations.chunk:
+ aggregations.annotations = await annotations.to_dict(
+ cast("DataStore", self)
+ )
+
+ references = await self._main_store.get_relations_for_event(
+ event_id, event, room_id, RelationTypes.REFERENCE, direction="f"
+ )
+ if references.chunk:
+ aggregations.references = await references.to_dict(cast("DataStore", self))
+
+ # Store the bundled aggregations in the event metadata for later use.
+ return aggregations
+
+ async def get_bundled_aggregations(
+ self, events: Iterable[EventBase], user_id: str
+ ) -> Dict[str, BundledAggregations]:
+ """Generate bundled aggregations for events.
+
+ Args:
+ events: The iterable of events to calculate bundled aggregations for.
+ user_id: The user requesting the bundled aggregations.
+
+ Returns:
+ A map of event ID to the bundled aggregation for the event. Not all
+ events may have bundled aggregations in the results.
+ """
+ # De-duplicate events by ID to handle the same event requested multiple times.
+ #
+ # State events do not get bundled aggregations.
+ events_by_id = {
+ event.event_id: event for event in events if not event.is_state()
+ }
+
+ # event ID -> bundled aggregation in non-serialized form.
+ results: Dict[str, BundledAggregations] = {}
+
+ # Fetch other relations per event.
+ for event in events_by_id.values():
+ event_result = await self._get_bundled_aggregation_for_event(event, user_id)
+ if event_result:
+ results[event.event_id] = event_result
+
+ # Fetch any edits (but not for redacted events).
+ edits = await self._main_store.get_applicable_edits(
+ [
+ event_id
+ for event_id, event in events_by_id.items()
+ if not event.internal_metadata.is_redacted()
+ ]
+ )
+ for event_id, edit in edits.items():
+ results.setdefault(event_id, BundledAggregations()).replace = edit
+
+ # Fetch thread summaries.
+ summaries = await self._main_store.get_thread_summaries(events_by_id.keys())
+ # Only fetch participated for a limited selection based on what had
+ # summaries.
+ participated = await self._main_store.get_threads_participated(
+ [event_id for event_id, summary in summaries.items() if summary], user_id
+ )
+ for event_id, summary in summaries.items():
+ if summary:
+ thread_count, latest_thread_event, edit = summary
+ results.setdefault(
+ event_id, BundledAggregations()
+ ).thread = _ThreadAggregation(
+ latest_event=latest_thread_event,
+ latest_edit=edit,
+ count=thread_count,
+ # If there's a thread summary it must also exist in the
+ # participated dictionary.
+ current_user_participated=participated[event_id],
+ )
+
+ return results
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