finance_orders.rb

ActiveAdmin.register Order, as: 'FinanceOrders' do
  idx = 0

  index do |x|
    column :id do
      idx += 1
    end
    column :contractor do |order|
      amount = order.contractor_payment_amount
      Finances.add amount
      amount.to_money
    end

    summary = Finances.get_summary collection

    tfoot do
      tr do
            column :contractor do |order|
              Finances.total.to_money
            end
            column :profit do |order|
              (order.sum_cost / 100).to_money * (1 - FEE) - Finances.total.to_money
            end
          end
        end
        td
      end
    end

  end

end

Finances.rb

class Finances

  @@sum = 0

  def self.get_orders
    Order.where(state: [1, 2, 3, 4])
  end

  def self.get_bids
    BidRequest.where(state: [1, 2, 3, 5])
  end

  def self.get_summary(collection)
    collection.select('*, sum(contractor_tip_cents) sum_tips, sum(total_cost_cents) sum_cost')
  end

  def self.add(amount)
    @@sum += amount
  end

  def self.total
    @@sum
  end

end