Add an action to add author to relative author's alias registry.

test/decentralised_book_index/metadata/author_test.exs

@@ -14,5 +14,19 @@ defmodule DecentralisedBookIndex.Metadata.AuthorTest do
       assert {:ok, author} = Metadata.create_author("Author", "An description", registry.id)
       assert author.author_alias_registry_id == registry.id
     end
+
+    test "a new author to a registry via a related author record" do
+      {:ok, related_author}  = Metadata.create_author("Author",  "An description")
+
+      assert {:ok, author} =
+        Metadata.add_author_to_related_alias_registry("Author2", "An description2", related_author.id)
+
+      assert related_author.author_alias_registry_id == author.author_alias_registry_id
+    end
+
+    test "a new author to a registry via a empty related author record" do
+      assert {:error, _} =
+        Metadata.add_author_to_related_alias_registry("Author2", "An description2", nil)
+    end
   end
 end