Add the action to read AuthorRoles by a Book id.

lib/decentralised_book_index/metadata/author_role.ex

@@ -39,6 +39,11 @@ defmodule DecentralisedBookIndex.Metadata.AuthorRole do
       filter expr(id == ^arg(:id))
     end
 
+    read :by_book_id do
+      argument :book_id, :uuid, allow_nil?: false
+      filter expr(book_id == ^arg(:book_id))
+    end
+
     update :update do
       primary? true
       accept [:order, :role]