Make formtastic's cancel action return the user to the original referrer

formtastic_cancel_action.rb

# Make formtastic's cancel action return the user to the original referrer
#
# By default, the cancel action in formtastic always goes to the previous
# page, so hitting cancel after validation failures returns the user to
# the form, not the original referring page. The code below modifies the
# cancel action such that it by default returns to the original referrer.
# It does so by simply keeping track of the referrer in a hidden field.
#
# See also https://github.com/justinfrench/formtastic/issues/780

require 'formtastic'

Formtastic::Actions::LinkAction.class_eval do

  def to_html
    wrapper do
      hidden_field_html + template.link_to(text, url, button_html)
    end
  end

  def url
    return options[:url] if options.key?(:url)
    hidden_field_value || :back
  end

  private
  def hidden_field_name
    "#{object_name}_#{method}_action_url"
  end

  def hidden_field_value
    return unless template.respond_to?(:params) && template.params[hidden_field_name].present?
    deserialize_url(template.params[hidden_field_name])
  end

  def hidden_field_html
    # note: encode url when writing to hidden form field to avoid false positives for remote file inclusion attacks
    return "".html_safe if options.key?(:url)
    template.hidden_field_tag(hidden_field_name, serialize_url(template.url_for(url)))
  end

  def serialize_url(value)
    # note: encode url as cgi query string to avoid false positives for xss attacks when using json
    ActiveSupport::JSON.decode(URI.parse(value).to_json).to_query
  end

  def deserialize_url(value)
    # build url from cgi query value
    URI::Generic.build(
      CGI.parse(value).tap do |h|
        # CGI.parse generates an array of values for each key, we just want the first
        h.each{|k,v| h[k] = v.first}
        # convert strings that represent integers to integers (e.g. port number)
        h.each{|k,v| h[k] = v.to_i if v.match(/^[0-9]+$/)}
        # convert empty strings to nil
        h.each{|k,v| h[k] = nil if v.blank?}
      end.symbolize_keys
    ).to_s
  end

end