gistfile1.rb

  # quantity isn't an attribute of the current model, so it doesn't show up in results
  named_scope :distinct_for_invoice, lambda {|ids| {
      :select => 'count(id) as quantity, product_id, net_price_in_cents',
      :conditions => ["id IN (?)", ids],
      :group => 'product_id, net_price_in_cents'
    }
  }