Bulk fetching

bulk_fetcher.rb

# This is an in-memory cache that can do bulk fetches (via find)
#   You can queue up ids to fetch at a later time.
class BulkFetcher

  def initialize
    reset!
  end

  def queue(klass, ids)
    @queue[klass] += Array(ids)
  end

  # returns objects by id
  def fetch_all(klass, id_or_ids)
    ids = Array(id_or_ids)

    # fetch anything queued for this class
    fetch_items(klass, @queue.delete(klass))

    # look in the cache for hits
    items, missing_ids = lookup_items(klass, ids)

    # bulk fetch the missing ids
    items += fetch_items(klass, missing_ids)

    items.group_by(&:id)
  end

  # return a single object
  def fetch(klass, id)
    fetch_all(klass, id).fetch(id)
  end

  def reset!
    @cache = {}
    @queue = Hash.new { [] }    
  end

  protected

  def lookup_items(klass, ids)
    missing = []
    items = []

    # find cache hits first
    ids.each do |id|
      if item = @cache.fetch(klass.name, {})[id]
        items.push(item)
      else
        missing.push(id)
      end
    end
    [items, missing]
  end

  def fetch_items(klass, ids)
    return [] if ids.blank?

    store_items(klass, klass.find(ids).to_a)
  end

  def store_items(klass, objects)
    objects.each do |object|
      @cache[klass.name] ||= {}
      @cache[klass.name][object.id] = object
    end

    objects
  end

end