def invalidate(strategy, value = nil)
      (value = strategy; strategy = :id) unless strategy.is_a?(Symbol)
      # call the invalidate method of the chosen strategy
      @strategy_by_attribute[strategy].invalidate(value) if @strategy_by_attribute[strategy]
    end
def invalidate(strategy, value = nil)
      (value = strategy; strategy = :id) unless strategy.is_a?(Symbol)
      # call the invalidate method of the chosen strategy
      @strategy_by_attribute[strategy].invalidate(value) if @strategy_by_attribute[strategy]
    end
Explicitly invalidate one or more records
 @param: strategy: the id of the strategy to invalidate (defaults to +:id+)
 @param: value: the value to send to the invalidate method of the chosen strategy