Cached hash

cached_hash.rb

class CachedHash
  extend ActiveSupport::Memoizable

  def initialize(cache_key)
    @cache_key = cache_key
  end

  def [](key)
    fetch[key]
  end

  def []=(key, value)
    value.tap { |value| unfrozen[key] = value; write }
  end

  def values
    fetch.values
  end

  def clear
    {}.tap { |hash| write(hash) }
  end

  def delete(key)
    unfrozen.delete(key).tap { write }
  end

  def size
    fetch.size
  end

private

  def cache_key
    "CACHED_HASH:#{@cache_key}"
  end
  memoize :cache_key

  def write(hash = @hash)
    Rails.cache.write(cache_key, hash || {})
  end

  def fetch
    @hash = Rails.cache.fetch(cache_key) { {} }
  end

  def unfrozen
    @hash = fetch.dup
  end

end

cached_hash_spec.rb

require 'spec_helper'

describe "a hash backed by a cache" do
  subject { CachedHash.new('some_key') }
  before(:each) { subject.clear }
  context "hash-like methods" do
    its(:size) { should == 0 }
    it "should be able to store a value under a key" do
      subject['foo'] = 'bar'
      subject['foo'].should == 'bar'
      subject.size.should == 1
    end
    it "should be able to get all of the values" do
      subject['foo'] = 'bar'
      subject['boo'] = 'far'
      subject.values.should == [ 'bar', 'far' ]
    end
    it "should be able to clear the hash" do
      subject['foo'] = 'bar'
      subject.clear.should == {}
      subject.size.should == 0
    end
    it "should be able to delete a key/value pair" do
      subject['foo'] = 'bar'
      subject.delete('foo').should == 'bar'
      subject.size.should == 0
    end
    it "should be able to return the number of entries" do
      subject['foo'] = 'bar'
      subject['boo'] = 'far'
      subject['The Answer'] = 42
      subject.size.should == 3
    end
  end
  context "cache-like features" do
    let!(:other_with_same_cache_key) { CachedHash.new('some_key') }
    it "should be able to find the same cached information via a different instance" do
      subject['foo'] = 'bar'
      subject.values.should == other_with_same_cache_key.values
    end
  end
end