fields_for_nested_model

# File actionview/lib/action_view/helpers/form_helper.rb, line 1918
        def fields_for_nested_model(name, object, fields_options, block)
          object = convert_to_model(object)
          emit_hidden_id = object.persisted? && fields_options.fetch(:include_id) {
            options.fetch(:include_id, true)
          }

          @template.fields_for(name, object, fields_options) do |f|
            output = @template.capture(f, &block)
            output.concat f.hidden_field(:id) if output && emit_hidden_id && !f.emitted_hidden_id?
            output
          end
        end