Question

Let’s suppose I have a method like this:

  def foo
    Rails.cache.fetch("cache_key", :expires_in => 60.minutes) do
      return_something
    end
  end

return_something sometimes returns a nil value. When this happens, I don’t want the nil value to be cached for 60 minutes. Instead, the next time I call foo, I want the block passed to fetch to be executed again.

Is Rails.cache.fetch working like this by default? Or do I have to implement this functionality?

Update (with Answer)

Turns out, the answer was no, at least when using Memcached.

Answer 1

it depends on the implementation of the cache-store that you are using. i would say that it should not cache nil values, but empty strings are ok to cache.

look at the dalli store implementation ie:

  def fetch(name, options=nil)
    options ||= {}
    name = expanded_key name

    if block_given?
      unless options[:force]
        entry = instrument(:read, name, options) do |payload|
          payload[:super_operation] = :fetch if payload
          read_entry(name, options)
        end
      end

      if !entry.nil?
        instrument(:fetch_hit, name, options) { |payload| }
        entry
      else
        result = instrument(:generate, name, options) do |payload|
          yield
        end
        write(name, result, options)
        result
      end
    else
      read(name, options)
    end
  end