sales_totals.rb

class SalesTotals
  include DataMapper::Resource
  include DataMapper::Caching
  
  property :id, Serial
  property :sales, Integer
  property :items, Integer
  property :value, Money
  property :start_at, Time
  property :end_at, Time
  
  calculate :sales do |records|
    records.length
  end
  
  calculate :items do |records|
    records.inject(0) do |memo, record|
      memo += record.items.count
      memo
    end
  end
end