Refactor query to return only 1 message instead of 20

lib/pleroma/conversation/participation.ex

@@ -65,22 +65,13 @@ def for_user(user, params \\ %{}) do
 
   def for_user_with_last_activity_id(user, params \\ %{}) do
     for_user(user, params)
-    |> Repo.preload(:conversation)
     |> Enum.map(fn participation ->
-      # TODO: Don't load all those activities, just get the most recent
-      # Involves splitting up the query.
-      activities =
-        ActivityPub.fetch_activities_for_context(participation.conversation.ap_id, %{
+      activity_id =
+        ActivityPub.fetch_latest_activity_id_for_context(participation.conversation.ap_id, %{
           "user" => user,
           "blocking_user" => user
         })
 
-      activity_id =
-        case activities do
-          [activity | _] -> activity.id
-          _ -> nil
-        end
-
       %{
         participation
         | last_activity_id: activity_id

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -459,35 +459,44 @@ def flag(
     end
   end
 
-  def fetch_activities_for_context(context, opts \\ %{}) do
+  defp fetch_activities_for_context_query(context, opts) do
     public = ["https://www.w3.org/ns/activitystreams#Public"]
 
     recipients =
       if opts["user"], do: [opts["user"].ap_id | opts["user"].following] ++ public, else: public
 
-    query = from(activity in Activity)
-
-    query =
-      query
+    from(activity in Activity)
     |> restrict_blocked(opts)
     |> restrict_recipients(recipients, opts["user"])
-
-    query =
-      from(
-        activity in query,
-        where:
+    |> where(
+      [activity],
       fragment(
         "?->>'type' = ? and ?->>'context' = ?",
         activity.data,
         "Create",
         activity.data,
         ^context
-          ),
-        order_by: [desc: :id]
       )
-      |> Activity.with_preloaded_object()
+    )
+    |> order_by([activity], desc: activity.id)
+  end
 
-    Repo.all(query)
+  @spec fetch_activities_for_context(String.t(), keyword() | map()) :: [Activity.t()]
+  def fetch_activities_for_context(context, opts \\ %{}) do
+    context
+    |> fetch_activities_for_context_query(opts)
+    |> Activity.with_preloaded_object()
+    |> Repo.all()
+  end
+
+  @spec fetch_latest_activity_id_for_context(String.t(), keyword() | map()) ::
+          Pleroma.FlakeId.t() | nil
+  def fetch_latest_activity_id_for_context(context, opts \\ %{}) do
+    context
+    |> fetch_activities_for_context_query(opts)
+    |> limit(1)
+    |> select([a], a.id)
+    |> Repo.one()
   end
 
   def fetch_public_activities(opts \\ %{}) do