Conditional GET helper for a RESTful API

api_conditional_get_helper.rb

# Written by https://github.com/macreery (c) Art.sy, 2012, MIT License

module ApiConditionalGetHelper
  # Borrows generously from http://themomorohoax.com/2009/01/07/using-stale-with-rails-to-return-304-not-modified
  # See also RFC 2616: http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.3.4
  #   for explanation of how If-Modified-Since and If-None-Match request headers are handled.
  
  def fresh?(metadata = {})
    self.last_modified = metadata[:last_modified]
    self.etag = metadata[:etag]
    case
    when if_modified_since && if_none_match
      not_modified?(metadata[:last_modified]) && etag_matches?(metadata[:etag])
    when if_modified_since
      not_modified?(metadata[:last_modified])
    when if_none_match
      etag_matches?(metadata[:etag])
    else
      false
    end
  end
    
  def if_modified_since
    if since = env["HTTP_IF_MODIFIED_SINCE"]
      Time.rfc2822(since) rescue nil
    end
  end

  def if_none_match
    env["HTTP_IF_NONE_MATCH"]
  end

  def not_modified?(modified_at)
    if_modified_since && modified_at && if_modified_since >= modified_at
  end

  def etag_matches?(etag)
    if_none_match && if_none_match == etag
  end
  
  def last_modified=(utc_time)
    return unless utc_time
    header "Last-Modified", utc_time.httpdate
  end
  
  def etag=(etag)
    return unless etag
    header "ETag", etag
  end
  
end