Update

lib/nulla_web/controllers/note_controller.ex

@@ -1,8 +1,9 @@
 defmodule NullaWeb.NoteController do
   use NullaWeb, :controller
-
-  alias Nulla.Notes
+  alias Nulla.Repo
   alias Nulla.Models.Note
+  alias Nulla.Models.InstanceSettings
+  alias Nulla.ActivityPub
 
   def index(conn, _params) do
     notes = Notes.list_notes()
@@ -26,11 +27,33 @@ defmodule NullaWeb.NoteController do
     end
   end
 
-  def show(conn, %{"id" => id}) do
-    note = Notes.get_note!(id)
-    render(conn, :show, note: note)
+  def show(conn, %{"username" => username, "note_id" => note_id}) do
+    accept = List.first(get_req_header(conn, "accept"))
+    instance_settings = InstanceSettings.get_instance_settings!()
+    domain = instance_settings.domain
+    note = Note.get_note!(note_id) |> Repo.preload([:user, :media_attachments])
+
+    if username != note.user.username do
+      conn
+      |> put_status(:not_found)
+      |> json(%{error: "Note not found"})
+      |> halt()
+    end
+
+    if accept in ["application/activity+json", "application/ld+json"] do
+      conn
+      |> put_resp_content_type("application/activity+json")
+      |> json(ActivityPub.note(domain, note))
+    else
+      render(conn, :show, domain: domain, note: note, layout: false)
+    end
   end
 
+  #def show(conn, %{"id" => id}) do
+  #  note = Notes.get_note!(id)
+  #  render(conn, :show, note: note)
+  #end
+
   def edit(conn, %{"id" => id}) do
     note = Notes.get_note!(id)
     changeset = Notes.change_note(note)