Rails per-request hash?

Question

Is there a way to cache per-request data in Rails? For a given Rails/mongrel request I have the result of a semi-expensive operation that I'd like to access several times later in that request. Is there a hash where I can store and access such data?

It needs to be fairly global and accessible from views, controllers, and libs, like Rails.cache and I18n are.

I'm ok doing some monkey-patching if that's what it takes.

• Memcached doesn't work because it'll be shared across requests, which I don't want.
• A global variable similarly doesn't work because different requests would share the same data, which isn't what I want.
• Instance variables don't work because I want to access the data from inside different classes.

Answer 1

Global variables are evil. Work out how to cleanly pass the data you want to where you want to use it.

Answer 2

Have you considered flash? It uses Session but is automatically cleared.

Answer 3

app/models/my_cacher.rb

class MyCacher
  def self.result
    @@result ||= begin
      # do expensive stuff
      # and cache in @@result
    end
  end
end

The ||= syntax basically means "do the following if @@result is nil" (i.e. not set to anything yet). Just make sure the last line in the begin/end block is returning the result.

Then in your views/models/whatever you would just reference the function when you need it:

MyCacher.result

This will cache the expensive action for the duration of a request.

Answer 4

Memoisation?

According to this railscast it's stored per request.

Answer 5

Try PerRequestCache. I stole the design from the SQL Query Cache.

Configure it up in config/environment.rb with:

config.middleware.use PerRequestCache

then use it with:

PerRequestCache.fetch(:foo_cache){ some_expensive_foo }