Add the action to get author's ids.

test/decentralised_book_index/metadata/author_test.exs

@@ -47,4 +47,27 @@ defmodule DecentralisedBookIndex.Metadata.AuthorTest do
       assert alternatives_names = [related_author]
     end
   end
+
+  describe "author's ids" do
+    test "the list has the author's id" do
+      {:ok, author} = Metadata.create_author("Author", "An description")
+
+      assert {:ok, ids} = Metadata.get_author_ids(author)
+      assert author.id in ids
+    end
+
+    test "the list has the aliases' ids" do
+      {:ok, author} = Metadata.create_author("Author", "An description")
+      {:ok, alias1} =
+        Metadata.add_author_to_related_alias_registry("Author2", "An description2", author.id)
+      {:ok, alias2} =
+        Metadata.add_author_to_related_alias_registry("Author3", "An description3", author.id)
+
+      assert {:ok, ids} = Metadata.get_author_ids(author)
+
+      assert author.id in ids
+      assert alias1.id in ids
+      assert alias2.id in ids
+    end
+  end
 end