Fix page ordering (#535)

api/pagination.py

@@ -216,57 +216,45 @@ class MastodonPaginator:
         max_id: str | None,
         since_id: str | None,
         limit: int | None,
+        home: bool = False,
     ) -> PaginationResult[TM]:
-        # These "does not start with interaction" checks can be removed after a
-        # couple months, when clients have flushed them out.
-        if max_id and not max_id.startswith("interaction"):
-            queryset = queryset.filter(id__lt=max_id)
-        if since_id and not since_id.startswith("interaction"):
-            queryset = queryset.filter(id__gt=since_id)
-        if min_id and not min_id.startswith("interaction"):
-            # Min ID requires items _immediately_ newer than specified, so we
-            # invert the ordering to accommodate
-            queryset = queryset.filter(id__gt=min_id).order_by("id")
-        else:
-            queryset = queryset.order_by("-id")
-
         limit = min(limit or self.default_limit, self.max_limit)
-        return PaginationResult(
-            results=list(queryset[:limit]),
-            limit=limit,
-        )
-
-    def paginate_home(
-        self,
-        queryset,
-        min_id: str | None,
-        max_id: str | None,
-        since_id: str | None,
-        limit: int | None,
-    ) -> PaginationResult:
-        """
-        The home timeline requires special handling where we mix Posts and
-        PostInteractions together.
-        """
+        filters = {}
+        id_field = "id"
+        reverse = False
+        if home:
+            # The home timeline interleaves Post IDs and PostInteraction IDs in an
+            # annotated field called "subject_id".
+            id_field = "subject_id"
             queryset = queryset.annotate(
-            event_id=Case(
+                subject_id=Case(
                     When(type=TimelineEvent.Types.post, then=F("subject_post_id")),
                     default=F("subject_post_interaction"),
                 )
             )
+
+        # These "does not start with interaction" checks can be removed after a
+        # couple months, when clients have flushed them out.
         if max_id and not max_id.startswith("interaction"):
-            queryset = queryset.filter(event_id__lt=max_id)
+            filters[f"{id_field}__lt"] = max_id
         if since_id and not since_id.startswith("interaction"):
-            queryset = queryset.filter(event_id__gt=since_id)
+            filters[f"{id_field}__gt"] = since_id
         if min_id and not min_id.startswith("interaction"):
             # Min ID requires items _immediately_ newer than specified, so we
             # invert the ordering to accommodate
-            queryset = queryset.filter(event_id__gt=min_id).order_by("event_id")
-        else:
-            queryset = queryset.order_by("-event_id")
+            filters[f"{id_field}__gt"] = min_id
+            reverse = True
+
+        # Default is to order by ID descending (newest first), except for min_id
+        # queries, which should order by ID for limiting, then reverse the results to be
+        # consistent. The clearest explanation of this I've found so far is this:
+        # https://mastodon.social/@Gargron/100846335353411164
+        ordering = id_field if reverse else f"-{id_field}"
+        results = list(queryset.filter(**filters).order_by(ordering)[:limit])
+        if reverse:
+            results.reverse()
 
-        limit = min(limit or self.default_limit, self.max_limit)
         return PaginationResult(
-            results=list(queryset[:limit]),
+            results=results,
             limit=limit,
         )

api/views/timelines.py

@@ -1,7 +1,7 @@
 from django.http import HttpRequest
 from hatchway import ApiError, ApiResponse, api_view
 
-from activities.models import Post
+from activities.models import Post, TimelineEvent
 from activities.services import TimelineService
 from api import schemas
 from api.decorators import scope_required
@@ -34,12 +34,13 @@ def home(
         "subject_post_interaction__post__mentions__domain",
         "subject_post_interaction__post__author__posts",
     )
-    pager = paginator.paginate_home(
+    pager: PaginationResult[TimelineEvent] = paginator.paginate(
         queryset,
         min_id=min_id,
         max_id=max_id,
         since_id=since_id,
         limit=limit,
+        home=True,
     )
     return PaginatingApiResponse(
         schemas.Status.map_from_timeline_event(pager.results, request.identity),