Add the validation for Book's page count to be greater than 0.

lib/decentralised_book_index/metadata/book.ex

@@ -302,6 +302,10 @@ defmodule DecentralisedBookIndex.Metadata.Book do
     end
   end
 
+  validations do
+    validate numericality(:page_count, greater_than: 0)
+  end
+
   relationships do
     belongs_to :dbi_server, Metadata.DBIServer