Update the create action for Author to set AuthorAliasRegistry.

test/decentralised_book_index/metadata/author_test.exs

@@ -0,0 +1,18 @@
+defmodule DecentralisedBookIndex.Metadata.AuthorTest do
+  use DecentralisedBookIndex.DataCase, async: true
+
+  alias DecentralisedBookIndex.Metadata
+
+  describe "authors and registries relationship" do
+    test "a new author get new registry by default" do
+      assert {:ok, author} = Metadata.create_author("Author", "An description")
+      assert author.author_alias_registry_id != nil
+    end
+
+    test "a new author belongs to a registry if specified" do
+      assert {:ok, registry} = Metadata.create_author_alias_registry()
+      assert {:ok, author} = Metadata.create_author("Author", "An description", registry.id)
+      assert author.author_alias_registry_id == registry.id
+    end
+  end
+end