Update the create action for Author to set AuthorAliasRegistry.

lib/decentralised_book_index/metadata.ex

@@ -6,7 +6,7 @@ defmodule DecentralisedBookIndex.Metadata do
     resource DecentralisedBookIndex.Metadata.Book
 
     resource DecentralisedBookIndex.Metadata.Author do
-      define :create_author, action: :create, args: [:name, :description, :author_alias_registry_id]
+      define :create_author, action: :create, args: [:name, :description, {:optional, :author_alias_registry_id}]
       define :add_author_to_alias_registry, action: :add_author_to_alias_registry, args: [:author_alias_registry_id]
       define :list_authors, action: :read
       define :get_author_by_id, args: [:id], action: :by_id

lib/decentralised_book_index/metadata/author.ex

@@ -20,6 +20,25 @@ defmodule DecentralisedBookIndex.Metadata.Author do
       primary? true
       accept [:name, :description, :author_alias_registry_id]
 
+      change fn changeset, _ ->
+        registry_id = Ash.Changeset.get_attribute(changeset, :author_alias_registry_id)
+
+        if registry_id == nil do
+          {:ok, registry} = DecentralisedBookIndex.Metadata.create_author_alias_registry()
+
+          Ash.Changeset.force_change_attribute(changeset, :author_alias_registry_id, registry.id)
+        else
+          changeset
+        end
+      end
+    end
+
+    read :by_id do
+      argument :id, :uuid, allow_nil?: false
+      get? true
+      filter expr(id == ^arg(:id))
+    end
+
     end
   end
 

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