foo.rb

class Foo < ActiveRecord::Base

  ALLOWED_HTML_ELEMENTS = %w[
    a b br blockquote code em h2 h3 hr i li ol p pre s strong sub sup u ul
  ]
  ALLOWED_EMPTY_HTML_ELEMENTS = %w[br hr]

  before_validation :sanitize_html

  private

  def sanitize_html
    unwanted_nodes = Loofah::Scrubber.new do |node|
      name = node.name.downcase
      node.remove unless (ALLOWED_HTML_ELEMENTS + ['text']).include?(name)
      node.remove if (node.blank? || node.content.blank?) && !ALLOWED_EMPTY_HTML_ELEMENTS.include?(name)
    end

    frag = Loofah.fragment(self.content).
             scrub!(:nofollow).
             scrub!(unwanted_nodes).
             scrub!(unwanted_nodes) # two-pass for catching empty elements after removing bad elements

    self.content = frag.to_html.squish.strip
  end

end