<%= form.fields_for :file_infos do |file_info_form| %> <% unless file_info_form.object.persisted? %> <%= file_info_form.hidden_field :filename, value: file_info_form.object.filename %> <%= file_info_form.hidden_field :byte_size, value: file_info_form.object.byte_size %> <%= file_info_form.hidden_field :content_type, value: file_info_form.object.content_type %> <% end %> <% end %>