conditional_get_collection.rb

# Conditional GET Support for a collection of ActiveRecord objects
#
# Useful in controllers when you want to support conditional get in
# index actions.
#
# example:
#   class PostsController
#     def index
#       if stale?(conditional_get_collection)
#         respond_with collection
#       end
#     end
#
#     def collection
#       current_user.posts
#     end
#
#     def conditional_get_collection
#       ConditionalGETCollection.new current_user: current_user,
#                                    collection: collection
#     end
#   end
class ConditionalGETCollection
  def initialize attributes = nil
    attributes ||= {}

    @collection   = attributes.fetch(:collection) { NullCollection.new }
    @hasher       = attributes.fetch(:hasher) { Digest::MD5 }
    @current_user = attributes[:current_user] || NullUser.new
  end

  def cache_key
    current_user_id     = @current_user.id
    seconds_since_epoch = updated_at.to_i
    count               = @collection.count

    @hasher.hexdigest [current_user_id, seconds_since_epoch, count].join('-')
  end

  def updated_at
    @collection.maximum(:updated_at)
  end

  class NullUser
    def id
      0
    end
  end

  class NullCollection
    def count
      0
    end

    def maximum attribute = :updated_at
      Time.new(1970, 1, 1, 0, 0, 0, 0).in_time_zone('UTC')
    end
  end
end