Update

lib/nulla_web/controllers/activitypub/note_controller.ex

@@ -0,0 +1,38 @@
+defmodule NullaWeb.ActivityPub.NoteController do
+  use NullaWeb, :controller
+  alias Nulla.Repo
+  alias NullaWeb.ActivityPub.NoteJSON
+  alias Nulla.Notes
+
+  def show(conn, %{"username" => username, "id" => id}) do
+    case Integer.parse(id) do
+      {int_id, ""} ->
+        note = Notes.get_note!(int_id) |> Repo.preload([:actor, :media_attachments])
+
+        cond do
+          is_nil(note) ->
+            conn
+            |> put_status(:not_found)
+            |> json(%{error: "Not Found"})
+            |> halt()
+
+          username != note.actor.preferredUsername ->
+            conn
+            |> put_status(:not_found)
+            |> json(%{error: "Not Found"})
+            |> halt()
+
+          true ->
+            conn
+            |> put_resp_content_type("application/activity+json")
+            |> json(NoteJSON.show(note))
+        end
+
+      _ ->
+        conn
+        |> put_status(:not_found)
+        |> json(%{error: "Not Found"})
+        |> halt()
+    end
+  end
+end