class ActiveSupport::Cache::MemCacheStore

def decrement(name, amount = 1, options = nil)

to zero.
Calling it on a value not stored with +:raw+ will initialize that value
operator and can only be used on values written with the +:raw+ option.
Decrement a cached value. This method uses the memcached decr atomic 
def decrement(name, amount = 1, options = nil)
  options = merged_options(options)
  instrument(:decrement, name, amount: amount) do
    rescue_error_with nil do
      @data.with { |c| c.decr(normalize_key(name, options), amount, options[:expires_in]) }
    end
  end
end