ActionController::ConditionalGet::ClassMethods
Methods
E
Instance Public methods
etag(&etagger)

Allows you to consider additional controller-wide information when generating an ETag. For example, if you serve pages tailored depending on who's logged in at the moment, you may want to add the current user id to be part of the ETag to prevent unauthorized displaying of cached pages.

class InvoicesController < ApplicationController
  etag { current_user.try :id }

  def show
    # Etag will differ even for the same invoice when it's viewed by a different current_user
    @invoice = Invoice.find(params[:id])
    fresh_when(@invoice)
  end
end

Source: show | on GitHub 

# File actionpack/lib/action_controller/metal/conditional_get.rb, line 29
def etag(&etagger)
  self.etaggers += [etagger]
end