Custom input field error for Rails forms

input_error.rb

ActionView::Base.field_error_proc = proc do |html_tag, instance_tag|
  fragment = Nokogiri::HTML.fragment(html_tag)
  field = fragment.at("input,select,textarea")

  html = if field
    field["class"] = "#{field["class"]} is-invalid"
    html = <<-HTML
      #{fragment}
      <p class="invalid-feedback">#{instance_tag.error_message.to_sentence}</p>
    HTML
    html
  else
    html_tag
  end

  html.html_safe
end