## model
class Configuration < ActiveRecord::Base
  class KeyNotFound < RuntimeError; end

  after_update :clear_cache

  # Reads config values. `Config["the_key"]`
  def self.[](key)
    Rails.cache.read("config::#{key}") || store_in_cache(key)
  end

  def self.store_in_cache(key)
    found_key = find_by_key(key)

    if found_key
      Rails.cache.write("config::#{key}", found_key.value)
      return Rails.cache.read("config::#{key}")
    else
      raise KeyNotFound, "Configuration key `#{key}' was not found."
    end
  end

  private

  def clear_cache
    Rails.cache.write("config:#{self.key}", nil)
  end
end

## test
require 'test_helper'

class ConfigurationTest < ActiveSupport::TestCase
  def setup
    Rails.cache.clear
  end

  test "reading directly from cache" do
    Rails.cache.write("config::foo", 5)
    Configuration.expects(:store_in_cache).never
    assert_equal 5, Configuration["foo"]
  end

  test "reading from db, storing in cache, and reading from cache" do
    Factory(:configuration, :key => "foo", :value => 5)
    assert_equal 5, Configuration["foo"]
  end

  test "key not in db" do
    assert_raises(Configuration::KeyNotFound) { Configuration["foo"] }
  end
end