Customize Field Errors with Rails 5 and Bootstrap

customize_error.rb

ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
  html = ''
  form_fields = %w(textarea input select)
  tag_elements = Nokogiri::HTML::DocumentFragment.parse(html_tag).css "label, " + form_fields.join(', ')

  tag_elements.each do |e|

    if e.node_name.eql? 'label'
      html = %(#{e}).html_safe
    elsif form_fields.include? e.node_name
      e['class'] = %(#{e['class']} is-invalid)
      if instance.error_message.kind_of?(Array)
        field_error_message = instance.error_message.uniq.join(', ')
      else
        field_error_message = instance.error_message
      end
      html = %(#{e}<div class="invalid-feedback">#{field_error_message}</div>).html_safe
    end
    
  end

  html

end