foo_sweeper.rb

class FooSweeper < ModelOnlySweeper
  
  observe Foo
  
  def after_save(object)
    expire_cache_for(object)
  end
  
  def after_destroy(object)
    expire_cache_for(object)
  end
  
private

  def expire_cache_for(object)
    base_key = Foo.some_cache_key  # For example.
    I18n.available_locales.each do |locale|  # Can't use regexp expiration with memcached.
      key = "#{base_key}:#{locale}"  # Probably generated by some shared method.
      expire_fragment(key)
    end  
  end
  
end

model_only_sweeper.rb

class ModelOnlySweeper < ActionController::Caching::Sweeper
  
  # We define FakeController for access to fragment_cache_key.
  # Usually Rails sweepers are aware of the current controller instance. We're trying to avoid that.
  # This means we have the benefit of not having to declare sweepers in controllers that update records,
  # but we can only use string keys, not hashes that would be used with url_for.
  module FakeController
    extend ActionController::Caching::Fragments
  end
  
private

  # When you use expire_fragment(key) from a sweeper, normally method_missing will pass it
  # on to the @controller. We don't have a @controller, so we define our own.
  def expire_fragment(key)
    if ActionController::Base.cache_configured?
      full_key = FakeController.fragment_cache_key(key)
      ActionController::Base.cache_store.delete(full_key)
    end    
  end
  
end