example_document.rb

class Product

  include MongoMapper::Document
  include MongoMapper::Sunspot

  belongs_to :crawl
  many :inventories

  key :product_no, Integer
  key :price_in_cents, Integer
  key :alcohol_content, Integer
  key :name, String
  key :stock_type, String
  key :primary_category, String
  key :secondary_category, String
  key :origin, String
  key :package, String
  key :sugar_content, String
  key :producer_name, String
  key :inventory_count, Integer

  ensure_index [:crawl_id, :product_no]

  sunspot do
    string :crawl_id
    integer :product_no
    string :name
    string :stock_type
    string :primary_category
    string :secondary_category
    string :origin
    string :package
    string :producer_name
  end

end

mongo_mapper_sunspot.rb

module MongoMapper
  module Sunspot

    def self.included(doc)
      doc.extend(ClassMethods)
    end

    module ClassMethods
      def sunspot(&block)
        Adapters.register(self)
        ::Sunspot.setup(self, &block)
      end
    end

    module Adapters
      def self.register(doc)
        ::Sunspot::Adapters::InstanceAdapter.register(InstanceAdapter, doc)
        ::Sunspot::Adapters::DataAccessor.register(DataAccessor, doc)
      end

      class InstanceAdapter < ::Sunspot::Adapters::InstanceAdapter
        def id
          @instance._id
        end
      end

      class DataAccessor < ::Sunspot::Adapters::DataAccessor
        def load(id)
          @clazz.find(id)
        end

        def load_all(ids)
          @clazz.find(*ids)
        end
      end
    end

  end
end

notes.txt

I have not tested any of this very much, but I will try to update
the gist if anything changes. I'm sure there is a nicer way to
integrate with MongoMapper somehow.