Starting a custom form builder in rails 6

application.rb

module SomeApp
  class Application < Rails::Application
    ...
    config.paths.add Rails.root.join('lib').to_s, autoload: true
  end
end

application_controller.rb

class ApplicationController < ActionController::Base
  default_form_builder ApplicationFormBuilder
end

application_form_builder.rb

# Starting a custom form builder (place in /lib)
# based on the rails view book (http://therailsview.com)

class ApplicationFormBuilder < ActionView::Helpers::FormBuilder
  def field_item(attribute, text=nil, &block)
    @template.content_tag :div, class: 'form-group' do
      @template.concat @template.label(attribute, (text.blank? ? attribute : text))
      yield
      @template.concat errors_on(attribute)
    end
  end

  def errors_on(attribute)
    if @object.errors[attribute].any?
      @template.content_tag(:span, 
                            object.errors[:attribute].to_sentence,
                            class: 'error')
    end
  end
end

field_error.rb

# place in config/initializers
ActionView::Base.field_error_proc = ->(field, instance) {field}