```ruby
module Machinist
  module Caching
    # The shop takes care of caching database objects.
    #
    # Calling make! on a class requests objects from the shop; you don't
    # normally access the shop directly.
    #
    # Read more about object caching on the
    # wiki[http://wiki.github.com/notahat/machinist/object-caching].
    class Shop
      # Return the singleton Shop instance.
      def self.instance
        @instance ||= Shop.new
      end

      def initialize #:nodoc:
        reset!
      end

      # Throw out the entire collection of cached objects.
      def reset!
        @warehouse = Warehouse.new
        restock
      end

      # Restock the shop with all the cached objects we've got.
      #
      # This should be called before each test.
      def restock
        @back_room = @warehouse.clone
      end

      # Buy a (possibly cached) object from the shop.
      #
      # This is just like constructing an object by calling Blueprint#make!,
      # but it will return a previously cached object if one is available.
      def buy(blueprint, attributes = {})
        raise BlueprintCantSaveError.new(blueprint) unless blueprint.respond_to?(:make!)

        shelf = @back_room[blueprint, attributes]
        if shelf.empty?
          object = blueprint.outside_transaction { blueprint.make!(attributes) }
          @warehouse[blueprint, attributes] << blueprint.box(object)
          object
        else
          blueprint.unbox(shelf.shift)
        end
      end
    end
  end
end
```

Use ruby singleton pattern instead of manual implementation
user
```ruby
require 'singleton'

module Machinist
  module Caching
    # The shop takes care of caching database objects.
    #
    # Calling make! on a class requests objects from the shop; you don't
    # normally access the shop directly.
    #
    # Read more about object caching on the
    # wiki[http://wiki.github.com/notahat/machinist/object-caching].
    class Shop
      include Singleton

      def initialize #:nodoc:
        reset!
      end

      # Throw out the entire collection of cached objects.
      def reset!
        @warehouse = Warehouse.new
        restock
      end

      # Restock the shop with all the cached objects we've got.
      #
      # This should be called before each test.
      def restock
        @back_room = @warehouse.clone
      end

      # Buy a (possibly cached) object from the shop.
      #
      # This is just like constructing an object by calling Blueprint#make!,
      # but it will return a previously cached object if one is available.
      def buy(blueprint, attributes = {})
        raise BlueprintCantSaveError.new(blueprint) unless blueprint.respond_to?(:make!)

        shelf = @back_room[blueprint, attributes]
        if shelf.empty?
          object = blueprint.outside_transaction { blueprint.make!(attributes) }
          @warehouse[blueprint, attributes] << blueprint.box(object)
          object
        else
          blueprint.unbox(shelf.shift)
        end
      end
    end
  end
end
```
assistant