Caching in Rails, Mysql, and Ubuntu 14.04

I have a Ruby on Rails application with a Mysql database running on Ubuntu 14.04. I have a query that is taking like 4 seconds to run. This is terrible so I’d like to cache it in rails.

I’m basically trying to do something like this:

def competing_price
    Rails.cache.fetch("#{cache_key}/competing_price", expires_in: 12.hours) do
      Competitor::API.find_price(id)
    end
  end

What can I do to implement this cache? I need either Memcached or Redis to act as the memory store. How do install this on DigitalOcean and then configure Rails to use this memory store? Got a tutorial?