diff --git a/lib/chiya/notes/note_note.ex b/lib/chiya/notes/note_note.ex
index a2ae964..2ccaf84 100644
--- a/lib/chiya/notes/note_note.ex
+++ b/lib/chiya/notes/note_note.ex
@@ -15,5 +15,6 @@ defmodule Chiya.Notes.NoteNote do
     note_note
     |> cast(attrs, [:source_id, :target_id])
     |> validate_required([:source_id, :target_id])
+    |> unique_constraint([:source_id, :target_id])
   end
 end