overriding Sunspot::Indexer.prepare to have different id field value

gistfile1.rb

# in Rails place this in config/initializers/sunspot_indexer_prepare.rb
Sunspot::Indexer.module_eval do
  alias :old_prepare :prepare

  def prepare(model)
    document = old_prepare(model)
    document.fields_by_name(:id).first.value = document.fields_by_name(:filename).first.value
    document
  end
end