gistfile1.rb

DataMapper.setup(:default, options = {}) # Your :default repo.
DataMapper.setup(:search, :adapter => 'sphinx')

class Item
  include DataMapper::Resource
  # .. normal properties and such for :default

  # This property is a computed field that only exists in :search aka your sphinx index{}.
  # They do not have to exist in :default.
  repository(:search) do
    property :my_extra_field, String
  end
end

# Search using extra field:
repository(:search) { Item.all(:my_extra_field => 'chicken') }

# Or if you are using dm-is-searchable:
Item.search(:my_extra_field => 'chicken')